cannot slice empty dataframe
Here is my test code:

```
import pandas as pd

print 'version', pd.__version__

df = pd.DataFrame({'x': []})
print df.iloc[0:0]
```

I would expect to get an empty dataframe. Instead I get an error:

```
version 0.13.1
Traceback (most recent call last):
  File "bug.py", line 6, in <module>
print df.iloc[0:0]
File "/usr/local/lib/python2.7/dist-packages/pandas/core/indexing.py", line 1028, in   __getitem__
    return self._getitem_axis(key, axis=0)
  File "/usr/local/lib/python2.7/dist-packages/pandas/core/indexing.py", line 1218, in _getitem_axis
    return self._get_slice_axis(key, axis=axis)
  File "/usr/local/lib/python2.7/dist-packages/pandas/core/indexing.py", line 1210, in _get_slice_axis
    typ='iloc')
  File "/usr/local/lib/python2.7/dist-packages/pandas/core/indexing.py", line 77, in _slice
    typ=typ)
  File "/usr/local/lib/python2.7/dist-packages/pandas/core/frame.py", line 1859, in _slice
    slobj, axis=axis, raise_on_error=raise_on_error)
  File "/usr/local/lib/python2.7/dist-packages/pandas/core/internals.py", line     2630, in get_slice
   _check_slice_bounds(slobj, new_axes[axis])
  File "/usr/local/lib/python2.7/dist-packages/pandas/core/indexing.py", line 1490, in _check_slice_bounds
    raise IndexError("out-of-bounds on slice (start)")
IndexError: out-of-bounds on slice (start)
```
