Integer partial slicing on multiindex bug
similar to one encountered in @wesm pydata 2013 tutorial

```
In [9]: import pandas as pd

In [10]: idx = pd.MultiIndex.from_arrays([[2011,2011,2012,2012,2013], [1,2,1,2,1]])

In [11]: ser = pd.Series(randn(len(idx)), idx)

In [12]: ser
Out[12]:
2011  1   -0.485310
      2   -0.679528
2012  1    1.089995
      2   -0.174895
2013  1   -1.219255
dtype: float64

In [13]: ser.ix[2011:2012]
Out[13]:
2011  1   -0.485310
      2   -0.679528
2012  1    1.089995
      2   -0.174895
dtype: float64

In [14]: ser[2011:2012]
Out[14]: Series([], dtype: float64)
```
