BUG: allow plot, boxplot, hist and completion on GroupBy objects
closes #5102 

also fixes extra plots created by groupby plotting (couldn't find an issue), now there's a test that ensures that groupby only plots the number of axes/figures it's supposed to
removes an unused internal groupby parameter

@jreback @jtratner thoughts?
 should those plot tests be marked slow?
 alright this is ready to go pending review, @jreback @jtratner ?
 good

maybe put a tab complete example for say frames in docs as well (basics?) later if u can
 done ... i put in `10min.rst` since tab completion is so useful that people should know about it as soon as they start using pandas.
 gre8...looks fine to me
 @nehalecky there you go :)
 Grand, thank you so much @cpcloud, this works perfectly! :)
