iloc with boolean mask
Currently masking by boolean vectors it doesn't matter which syntax you use:

```
df[mask]
df.iloc[mask]
df.loc[mask]
```

are all equivalent. Should mask `df.iloc[mask]` mask by position? (this makes sense if mask is integer index).

This [SO question](http://stackoverflow.com/questions/16603765/what-is-the-most-idiomatic-way-to-index-an-object-with-a-boolean-array-in-pandas).
