DOCS: Example landing page

Finally implemented a thumbnail gallery-style landing page for the docs. Inspired by nbviewers frontpage [1] and that there aren't any statsmodels notebooks there...

Only downside is, you have to make the thumbnails yourself, but it's not that tough to do. Every notebook example should have a "hook."

I've also organized the notebooks into respective categories, so we see where need more. I hope this inspires people to turn their scripts into examples. Or even to pull out nice examples from our docs and make example notebooks for the gallery.

[1] http://nbviewer.ipython.org/