.loc should accept out-of-bounds indexers for slices
pandas 0.14 [changed `.iloc`](http://pandas.pydata.org/pandas-docs/version/0.15.0/whatsnew.html#whatsnew-0140-api) to accept out-of-bounds indexers for slices. I think we should do the same for `.loc`, for similar reasons of consistency and efficiency.

Note that this is already currently inconsistent for int and float indexes (not entirely sure why):

```
>>> pd.Series(range(5), np.arange(5)).loc[-1:]
KeyError
>>> pd.Series(range(5), np.arange(5.0)).loc[-1:]
0    0
1    1
2    2
3    3
4    4
dtype: int64
```
