BUG: barplot with NaNs

Closes https://github.com/pydata/pandas/issues/8175

This makes barplot more consistent with how NaN's are handled in AreaPlots and PiePlots.

well, the user can always fill it first (that's what you are recommending in the docs). If you are going to do it by default, then the user should have the ability to turn it off I guess (0 does seem sensible). I think

``df.fillna(......).plot(kind='bar',fillna=None)`` is ok (if you really really don't want to fill it)
``df.plot(kind='bar',fllna=0)`` seems a good default though(but should be only for 'bar' right?)