ENH: allow legend='reverse' in df.plot() GH6014

![reverse](https://f.cloud.github.com/assets/1820866/2007133/009ee032-8732-11e3-920c-7bb53c754ed7.png)

works for line plots, bar plots, and kde plots. scatter plots don't seem to have a legend.

As usual, the plotting code is a tangled mess, I have no idea if this breaks something,
testing seems like a herculean task and I'm scared to death that by being brave and merging
this, I'm setting myself up to touch the plotting code again in the future. at least once. quite
the deterrent.

#6014