tail() fails on empty DataFrames
`tail()` method being called on empty DataFrame results in IndexError exception. At the same time `head()` works fine:

```
>>> df = pd.DataFrame()
>>> print df.head()
Empty DataFrame
Columns: []
Index: []

[0 rows x 0 columns]
>>> print df.tail()
Traceback (most recent call last):
  File "<stdin>", line 1, in <module>
  File "/Users/v/.virtual_envs/system/lib/python2.7/site-packages/pandas/core/generic.py", line 1572, in tail
    return self.iloc[-n:]
  File "/Users/v/.virtual_envs/system/lib/python2.7/site-packages/pandas/core/indexing.py", line 1020, in __getitem__
    return self._getitem_axis(key, axis=0)
  File "/Users/v/.virtual_envs/system/lib/python2.7/site-packages/pandas/core/indexing.py", line 1210, in _getitem_axis
    return self._get_slice_axis(key, axis=axis)
  File "/Users/v/.virtual_envs/system/lib/python2.7/site-packages/pandas/core/indexing.py", line 1202, in _get_slice_axis
    typ='iloc')
  File "/Users/v/.virtual_envs/system/lib/python2.7/site-packages/pandas/core/indexing.py", line 75, in _slice
    typ=typ)
  File "/Users/v/.virtual_envs/system/lib/python2.7/site-packages/pandas/core/frame.py", line 1836, in _slice
    slobj, axis=axis, raise_on_error=raise_on_error)
  File "/Users/v/.virtual_envs/system/lib/python2.7/site-packages/pandas/core/internals.py", line 2516, in get_slice
    _check_slice_bounds(slobj, new_axes[axis])
  File "/Users/v/.virtual_envs/system/lib/python2.7/site-packages/pandas/core/indexing.py", line 1523, in _check_slice_bounds
    raise IndexError("out-of-bounds on slice (start)")
IndexError: out-of-bounds on slice (start)
```

Version of pandas: 0.13.0
