Index.droplevel when names are tuples
#### Code Sample, a copy-pastable example if possible

```python
In [2]: mi = pd.MultiIndex.from_product([[0, 1]]*2, names=[('first', 'level'), ('second', 'level')])

In [3]: mi.droplevel(('first', 'level'))
---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
~/nobackup/repo/pandas/pandas/core/indexes/multi.py in _get_level_number(self, level)
    775                                  'level number' % level)
--> 776             level = self.names.index(level)
    777         except ValueError:

ValueError: 'first' is not in list

During handling of the above exception, another exception occurred:

KeyError                                  Traceback (most recent call last)
<ipython-input-3-1952401ac0fc> in <module>()
----> 1 mi.droplevel(('first', 'level'))

~/nobackup/repo/pandas/pandas/core/indexes/base.py in droplevel(self, level)
   3177             level = [level]
   3178 
-> 3179         levnums = sorted(self._get_level_number(lev) for lev in level)[::-1]
   3180 
   3181         if len(level) == 0:

~/nobackup/repo/pandas/pandas/core/indexes/base.py in <genexpr>(.0)
   3177             level = [level]
   3178 
-> 3179         levnums = sorted(self._get_level_number(lev) for lev in level)[::-1]
   3180 
   3181         if len(level) == 0:

~/nobackup/repo/pandas/pandas/core/indexes/multi.py in _get_level_number(self, level)
    777         except ValueError:
    778             if not isinstance(level, int):
--> 779                 raise KeyError('Level %s not found' % str(level))
    780             elif level < 0:
    781                 level += self.nlevels

KeyError: 'Level first not found'

In [4]: mi.droplevel([('first', 'level')])
Out[4]: Int64Index([0, 1, 0, 1], dtype='int64', name=('second', 'level'))
```

#### Problem description

Always the same story: if we allow users to pass tuples of stuff interchangeably with lists of stuff, then tuples as values (or names, as in this case) will cause problems.

#### Expected Output

``Out[4]``

... as long as we all agree that the ``level`` argument can accept lists of names but not tuples of names. The only other option I see is to document that ``droplevel`` does not support tuple names (except as items of a list).

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

<details>

INSTALLED VERSIONS
------------------
commit: None
python: 3.5.3.final.0
python-bits: 64
OS: Linux
OS-release: 4.9.0-6-amd64
machine: x86_64
processor: 
byteorder: little
LC_ALL: None
LANG: it_IT.UTF-8
LOCALE: it_IT.UTF-8

pandas: 0.24.0.dev0+14.g489cc24c8.dirty
pytest: 3.5.0
pip: 9.0.1
setuptools: 39.0.1
Cython: 0.25.2
numpy: 1.14.1
scipy: 0.19.0
pyarrow: None
xarray: None
IPython: 6.2.1
sphinx: 1.5.6
patsy: 0.5.0
dateutil: 2.7.0
pytz: 2017.2
blosc: None
bottleneck: 1.2.0dev
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: 1.3.0
xlsxwriter: 0.9.6
lxml: 4.1.1
bs4: 4.5.3
html5lib: 0.999999999
sqlalchemy: 1.0.15
pymysql: None
psycopg2: None
jinja2: 2.10
s3fs: None
fastparquet: None
pandas_gbq: None
pandas_datareader: 0.2.1

</details>
