BUG/VIS: fix Series.hist so that users can create hist subplots without the mpl API
closes #4408.
closes #4402.

~~ahhh nice, `matplotilb` has an image comparison decorator i'll use that~~

it's too specific to `matplotlib`'s testing suite.
 @jreback u ok with this? fixes a couple of issues with overlapping subplots when ppl want to use the matlab-like pylab stuff
 also passing both color and style when style has a color symbol is not valid anymore, not sure it ever was, since the ts plot tests pass but that's b/c the error is somewhere deep in the mpl stack and it isn't propagated up to pandas
 so not an API change since i don't think it ever worked, but pandas won't try to create a plot if those two args are invalid, it will raise instead
 ok w me
