Make partial slicing work with DatetimeIndex
e.g. `ts['2000':'2004']`

```
---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
/home/wesm/Dropbox/talks/notebooks/<ipython-input-31-6c807131b1d0> in <module>()
----> 1 close_px.ix['2000':'2003']

/home/wesm/code/pandas/pandas/core/indexing.pyc in __getitem__(self, key)
     33             return self._getitem_tuple(key)
     34         else:
---> 35             return self._getitem_axis(key, axis=0)
     36 
     37     def _get_label(self, label, axis=0):

/home/wesm/code/pandas/pandas/core/indexing.pyc in _getitem_axis(self, key, axis)
    165         labels = self.obj._get_axis(axis)
    166         if isinstance(key, slice):
--> 167             return self._get_slice_axis(key, axis=axis)
    168         elif _is_list_like(key) and not (isinstance(key, tuple) and
    169                                          isinstance(labels, MultiIndex)):

/home/wesm/code/pandas/pandas/core/indexing.pyc in _get_slice_axis(self, slice_obj, axis)
    361         else:
    362             try:
--> 363                 i, j = labels.slice_locs(start, stop)
    364                 slicer = slice(i, j, slice_obj.step)
    365             except Exception:

/home/wesm/code/pandas/pandas/core/index.pyc in slice_locs(self, start, end)
    909         else:
    910             try:
--> 911                 end_slice = self.get_loc(end) + 1
    912             except KeyError:
    913                 if self.is_monotonic:

TypeError: unsupported operand type(s) for +: 'slice' and 'int'
```
