DataFrame.loc throws AttributeError with rows of datetime objects
The following code:

```
import pandas as pd
from datetime import time
x = pd.DataFrame({'name': ['A', 'B', 'C', 'D'],
                  'time': [time(14, 12), time(12, 14), time(2, 45), time(5, 55)],
                  'category': ['X', 'X', 'Y', 'Y']})

y = x.set_index(['category', 'name']).transpose()

y.loc['time', ('X', 'A')]
```

Gives the following traceback:

```
Traceback (most recent call last):
  File "evil_loc.py", line 9, in <module>
    y.loc['time', ('X', 'A')]
  File "/users/is/whughes/pyenvs/73b3a6c24834a212/lib/python2.7/site-packages/pandas/core/indexing.py", line 1142, in __getitem__
    return self._getitem_tuple(key)
  File "/users/is/whughes/pyenvs/73b3a6c24834a212/lib/python2.7/site-packages/pandas/core/indexing.py", line 655, in _getitem_tuple
    return self._getitem_lowerdim(tup)
  File "/users/is/whughes/pyenvs/73b3a6c24834a212/lib/python2.7/site-packages/pandas/core/indexing.py", line 763, in _getitem_lowerdim
    return self._getitem_nested_tuple(tup)
  File "/users/is/whughes/pyenvs/73b3a6c24834a212/lib/python2.7/site-packages/pandas/core/indexing.py", line 844, in _getitem_nested_tuple
    if obj.ndim < current_ndim:
AttributeError: 'datetime.time' object has no attribute 'ndim'
```

This may only occur with numpy 1.7, I haven't managed to reproduce on 1.8 yet.
