Fix for DataFrame.hist() with by- and weights-keyword
will make the following work

``` python
import numpy as np
import pandas as pd
d = {'one' : ['A', 'A', 'B', 'B', 'C'],
     'two' : [4., 3., 2., 1., np.nan],
     'three' : [10., 8., np.nan, 5., 7.]}     
df = pd.DataFrame(d)
df.hist('two', by='one', weights='three', bins=range(0, 10))
# or
df.hist('two', by=df.one.values, weights='three', bins=range(0, 10))
```
