Facet plot

This is a rough implementation of the facet_plot by means of formulas, a-là-R.
The result is not yet production ready but is satisfying.

it takes a formula in the form
Y ~ X | factor
and a dataframe and plot automatically a correct (almost) plot given the datatype of the series. Handle without problem if some of the terms in the formula are missing.

It requires the mosaicplot function that I proposed in pull request 620 (https://github.com/statsmodels/statsmodels/pull/620).

All the documentation is still missing, but will be written inthe next few days
![facet_plot_example](https://f.cloud.github.com/assets/1419337/215362/b0faeb88-8481-11e2-97c6-2f0e9363f9e3.png)