Two fixes

This fixes two things:

* The title in facet plots is centered https://github.com/yhat/ggplot/issues/47#issuecomment-30340126
* Closes https://github.com/yhat/ggplot/issues/33 -> now legends show up in ipython

There should probably some unittets images updates, but currently my system rpduces not antialised images and so they will fail on systems which do (I traced that problem to a line in mathplotlibs tests setup, which ggplot uses).