support Ellipsis (...) in pandas indexing
Indexing like `x[..., 0]` is a convenient way to indexing the last dimension of an array with NumPy. `...` gets turned into an `Ellipsis` object, which NumPy expands to the appropriate number of full slice objects.

It would be nice to support this in pandas, too. Currently, attempting to do this throws KeyError:

```
>>> df[...]
KeyError: Ellipsis
```
