Remove bar line
Closes https://github.com/pydata/pandas/issues/9088
Closes https://github.com/pydata/pandas/issues/7128

Votes on whether we should
1. Remove the line entirely
2. Add a parameter to `.plot` control whether the line should be added.

I vote for 1, but I can go either way.

-1 in adding a parameter - this just looks like a bug
and could be done if the user really wants via matplotlib (the default is what u r doing as it seems more common)
 Agreed, let's kill it. It's easy enough to add in manually afterwards.
 minor doc issue. looks good.
 @TomAugspurger I think you can merge this
