AttributeError when slicing a datetime MultiIndex level
#### Code Sample, a copy-pastable example if possible

```python
In [2]: df = pd.read_csv(pd.__path__[0]+'/tests/tools/data/quotes2.csv', parse_dates=['time']).set_index(['ticker', 'time']).sort_index()

In [3]: df.loc['AAPL'].loc[slice('2016-05-25 13:30:00'), :].head()
Out[3]: 
                           bid    ask
time                                 
2016-05-25 13:30:00.075  98.55  98.56
2016-05-25 13:30:00.076  98.55  98.56
2016-05-25 13:30:00.076  98.55  98.56
2016-05-25 13:30:00.076  98.55  98.56
2016-05-25 13:30:00.080  98.55  98.56

In [4]: df.loc[('AAPL', slice('2016-05-25 13:30:00')), :].head()
---------------------------------------------------------------------------
AttributeError                            Traceback (most recent call last)
<ipython-input-4-552cfee2e501> in <module>()
----> 1 df.loc[('AAPL', slice('2016-05-25 13:30:00')), :].head()

/home/nobackup/repo/pandas/pandas/core/indexing.py in __getitem__(self, key)
   1322             except (KeyError, IndexError):
   1323                 pass
-> 1324             return self._getitem_tuple(key)
   1325         else:
   1326             key = com._apply_if_callable(key, self.obj)

/home/nobackup/repo/pandas/pandas/core/indexing.py in _getitem_tuple(self, tup)
    833     def _getitem_tuple(self, tup):
    834         try:
--> 835             return self._getitem_lowerdim(tup)
    836         except IndexingError:
    837             pass

/home/nobackup/repo/pandas/pandas/core/indexing.py in _getitem_lowerdim(self, tup)
    945         # we may have a nested tuples indexer here
    946         if self._is_nested_tuple_indexer(tup):
--> 947             return self._getitem_nested_tuple(tup)
    948 
    949         # we maybe be using a tuple to represent multiple dimensions here

/home/nobackup/repo/pandas/pandas/core/indexing.py in _getitem_nested_tuple(self, tup)
   1020 
   1021             current_ndim = obj.ndim
-> 1022             obj = getattr(obj, self.name)._getitem_axis(key, axis=axis)
   1023             axis += 1
   1024 

/home/nobackup/repo/pandas/pandas/core/indexing.py in _getitem_axis(self, key, axis)
   1542             # nested tuple slicing
   1543             if is_nested_tuple(key, labels):
-> 1544                 locs = labels.get_locs(key)
   1545                 indexer = [slice(None)] * self.ndim
   1546                 indexer[axis] = locs

/home/nobackup/repo/pandas/pandas/indexes/multi.py in get_locs(self, tup)
   2173                 # a slice, include BOTH of the labels
   2174                 indexer = _update_indexer(_convert_to_indexer(
-> 2175                     self._get_level_indexer(k, level=i, indexer=indexer)),
   2176                     indexer=indexer)
   2177             else:

/home/nobackup/repo/pandas/pandas/indexes/multi.py in _get_level_indexer(self, key, level, indexer)
   2054                 # note that the stop ALREADY includes the stopped point (if
   2055                 # it was a string sliced)
-> 2056                 return convert_indexer(start.start, stop.stop, step)
   2057 
   2058             elif level > 0 or self.lexsort_depth == 0 or step is not None:

AttributeError: 'int' object has no attribute 'start'
```
#### Problem description

The two calls should be perfectly equivalent.

By the way, if I swap the levels, I get a different error:

``` python
In [16]: df = pd.read_csv(pd.__path__[0]+'/tests/tools/data/quotes2.csv', parse_dates=['time']).set_index(['time', 'ticker']).sort_index()

In [26]: df.loc[:'2016-05-25 13:30:00'].head()
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
<ipython-input-26-a0298286991a> in <module>()
----> 1 df.loc[:'2016-05-25 13:30:00'].head()

/home/nobackup/repo/pandas/pandas/core/indexing.py in __getitem__(self, key)
   1325         else:
   1326             key = com._apply_if_callable(key, self.obj)
-> 1327             return self._getitem_axis(key, axis=0)
   1328 
   1329     def _is_scalar_access(self, key):

/home/nobackup/repo/pandas/pandas/core/indexing.py in _getitem_axis(self, key, axis)
   1503         if isinstance(key, slice):
   1504             self._has_valid_type(key, axis)
-> 1505             return self._get_slice_axis(key, axis=axis)
   1506         elif is_bool_indexer(key):
   1507             return self._getbool_axis(key, axis=axis)

/home/nobackup/repo/pandas/pandas/core/indexing.py in _get_slice_axis(self, slice_obj, axis)
   1353         labels = obj._get_axis(axis)
   1354         indexer = labels.slice_indexer(slice_obj.start, slice_obj.stop,
-> 1355                                        slice_obj.step, kind=self.name)
   1356 
   1357         if isinstance(indexer, slice):

/home/nobackup/repo/pandas/pandas/indexes/base.py in slice_indexer(self, start, end, step, kind)
   3226         """
   3227         start_slice, end_slice = self.slice_locs(start, end, step=step,
-> 3228                                                  kind=kind)
   3229 
   3230         # return a slice

/home/nobackup/repo/pandas/pandas/indexes/multi.py in slice_locs(self, start, end, step, kind)
   1741         # This function adds nothing to its parent implementation (the magic
   1742         # happens in get_slice_bound method), but it adds meaningful doc.
-> 1743         return super(MultiIndex, self).slice_locs(start, end, step, kind=kind)
   1744 
   1745     def _partial_tup_index(self, tup, side='left'):

/home/nobackup/repo/pandas/pandas/indexes/base.py in slice_locs(self, start, end, step, kind)
   3413         end_slice = None
   3414         if end is not None:
-> 3415             end_slice = self.get_slice_bound(end, 'right', kind)
   3416         if end_slice is None:
   3417             end_slice = len(self)

/home/nobackup/repo/pandas/pandas/indexes/multi.py in get_slice_bound(self, label, side, kind)
   1712         if not isinstance(label, tuple):
   1713             label = label,
-> 1714         return self._partial_tup_index(label, side=side)
   1715 
   1716     def slice_locs(self, start=None, end=None, step=None, kind=None):

/home/nobackup/repo/pandas/pandas/indexes/multi.py in _partial_tup_index(self, tup, side)
   1770                 start = start + section.searchsorted(idx, side='left')
   1771             else:
-> 1772                 return start + section.searchsorted(idx, side=side)
   1773 
   1774     def get_loc(self, key, method=None):

TypeError: unorderable types: int() > slice()
```

... while everything works fine if there is one level only.

#### Expected Output

The same as ``Out[3]:``.

#### Output of ``pd.show_versions()``

<details>

INSTALLED VERSIONS
------------------
commit: 4502e82083f4e253630588665a4fc6002c4f32ed
python: 3.5.3.final.0
python-bits: 64
OS: Linux
OS-release: 4.7.0-1-amd64
machine: x86_64
processor: 
byteorder: little
LC_ALL: None
LANG: it_IT.utf8
LOCALE: it_IT.UTF-8

pandas: 0.19.0+743.g4502e8208
pytest: 3.0.6
pip: 9.0.1
setuptools: 33.1.1
Cython: 0.25.2
numpy: 1.12.0
scipy: 0.18.1
xarray: 0.9.1
IPython: 5.1.0.dev
sphinx: 1.4.9
patsy: 0.3.0-dev
dateutil: 2.5.3
pytz: 2016.7
blosc: None
bottleneck: 1.2.0
tables: 3.3.0
numexpr: 2.6.1
feather: 0.3.1
matplotlib: 2.0.0
openpyxl: 2.3.0
xlrd: 1.0.0
xlwt: 1.1.2
xlsxwriter: 0.9.6
lxml: 3.7.1
bs4: 4.5.3
html5lib: 0.999999999
sqlalchemy: 1.0.15
pymysql: None
psycopg2: None
jinja2: 2.8
s3fs: None
pandas_gbq: None
pandas_datareader: 0.2.1


</details>
