Enrico facet plot

rebased version of Enrico's facet plot PR #692

Warning: the rebase needs to be redone and this branch will be abandoned

experimental branch (until I understand what's going on, let's see what Travis says)
(I'm not fully setup anymore to test quickly different python versions)

rebase looked good, no differences in facet plot files to original branch

some strange duplicate commit messages for mosaic plot
https://github.com/josef-pkt/statsmodels/commits/enrico_facet-plot?page=2
There was a mistake in the automatic rebase, all those changes apply to sandbox/mosaic_plots.py and its tests.
Those don't exist anymore in master.

I'm getting 15 errors and 2 failures on python 2.6, largely patsy formula parsing errors.

* Most tests are disabled with @nottest,
* tests include show that pops up plots
* test import pylab instead of pyplot (uses extra pylab functions)