MultiIndex loc for value not in index
example:

```
In [11]: s = pd.Series({(0, 0): 1, (0, 1): 2, (0, 3): 3, (1, 0): 1, (1, 2): 4, (3, 0): 5})

In [12]: s.loc[(1, 0):]
Out[12]:
(1, 0)    1
(1, 2)    4
(3, 0)    5
dtype: int64

In [13]: s.loc[(1, 1):]
KeyError: 'start bound [(1, 1)] is not the [index]'
```

Should this work? cc @jreback 

_From [SO question](http://stackoverflow.com/questions/18015094/slice-pandas-series-with-elements-not-in-the-index)._
