consistency of hist and boxplot
Both `DataFrame.hist` and `DataFrame.boxplot` support `by` to create grouped plots, however, `DataFrame.hist` requires that any column in `by` is also included in `column` while `DataFrame.boxplot` its vice-versa.

Here is a minimal example::

```
>>> df = pd.DataFrame({'a': [1, 1, 2, 2, 2, 1], 'b': list('AAABBB')})
>>> df.hist(column=['a', 'b'], by=['b'])
>>> df.boxplot(column=['a'], by=['b'])

>>> df.hist(column=['a'], by=['b'])  # raises an error
KeyError: u'no item named b'
```
