ENH: add figsize argument to DataFrame and Series hist methods
closes #3834 

@jreback ok with this?
 don't we always just pass kwds to matplotlib anyhow? (I am not disagreeing with this, because its nice
to do this on the front-end), just curios
 yes but they weren't being propagated correctly because they were being passed into funcs that don't use them e.g., grouped_hist. plus hist_series was calling gca which creates a figure before u can give it a figsize. u might be able to adjust that post creation but my method requires less mpl api munging
 oh i just saw something that i need to change can't merge yet..
 minor point, figsize is in inches I think? (subject to dpi), (may want to mention that)
 units there are customizable i believe but ok
 @jreback this is good 2 go...shall i merge?
 ok
