Using boolean Series to mask array broken in 0.13
Hi, I'm testing out some code on the 0.13 release candidate and I've run into problems with a fairly common (for me) pattern. It's no longer possible to use a boolean Series to index a numpy array. E.g.:

```
import numpy as np
import pandas as pd
x = np.random.randn(30)
mask = pd.Series(np.random.rand(30) > .5)
x[mask].mean()
```

Raises:

```
---------------------------------------------------------------------------
IndexError                                Traceback (most recent call last)
<ipython-input-27-73e40e756b4e> in <module>()
      3 x = np.random.randn(30)
      4 mask = pd.Series(np.random.rand(30) > .5)
----> 5 x[mask].mean()

IndexError: unsupported iterator index
```

Perhaps this is not a good idiom, but this change breaks quite a bit of existing code.
