API: MultiIndex.get_level_values should have label only / level only mode?
Currently, `MultiIndex.get_level_values` handles both names and levels, it return different levels depending on its name (like `.ix`). Adding an option (or separate method) like `.loc` and `.iloc` to avoid any unexpected results?

```
idx = pd.MultiIndex.from_tuples([(1, 'A'), (2, 'B')])
idx.get_level_values(0)
# Int64Index([1, 2], dtype='int64')

idx = idx.set_names([1, 0])
idx.get_level_values(0)
# Index([u'A', u'B'], dtype='object', name=0)
```
