consistency of hist and boxplot

[pprett]

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'

[pprett]

this might be a duplicate of #413

[jreback]

closing as already marked in #413 (still unresolved though)