tail throws exception on empty frame
currently, tail throws an exception on an empty frame but head does not.

```
import pandas as pd
df = pd.DataFrame()
df.tail()
```

traceback:
https://gist.github.com/MichaelWS/8256143

Should this be fixed by returning the frame if empty on calls to iloc?  

We can check for empty in tail on length as well or create a new slice with start being None)
