DataFrame.hist ignores keyword arguments
`DataFrame.hist` allows arbitrary keyword arguments which (according to the docstring) are passed to `matplotlib.hist`, however, it ignores keyword arguments in `pandas.tools.plotting.grouped_hist` (does not pass them to `hist`). 

This would be useful in order to create normalized and/or cumulative histograms. 
It requires just a minor fix - if you want I can submit a PR.
