Geom_boxplot: Started a rough outline of how geom_boxplot will be implemented.

Currently does nothing close to ggplot2's geom_boxplot().

I think this is ready to merge at this point if you want. It does work for a single boxplot, see the test case in test/test_geom_boxplot.py. There is one problem I ran into though. Is there anyway to access the whole dataframe that gets passed into ggplot(..) ? My new strategy for implementing the geom_boxplot is to use to http://pandas.pydata.org/pandas-docs/stable/visualization.html#box-plotting which is much more similar to ggplot2's than http://matplotlib.org/api/pyplot_api.html#matplotlib.pyplot.boxplot. Let me know your thoughts on this.