Add kind='line' or 'bar' argument to DataFrame.plot
This is just a first stab at it, comments are very welcome, plus feel free to say you'd rather not add it at all.

Thanks, Dieter. I refactored the method, tweaked, the plot a little bit, and added some unit tests. Can see this being very useful-- too bad matplotlib isn't better as making these kinds of plots without a lot of tweaking
 FYI you might want to fix up your master branch because I cherry-picked this commit on top of wesm/master 
