First pass at the reorganization of content on the matplotlib home page.

cherry-picked commit over to current 1.4.x, this replaces #3245

@nellev The current build of the docs should be at  http://tacaswell.github.io/matplotlib (eventually, waiting on github.io to sort it's self out as I write this)

@vestuto Please make sure that I didn't break anything during the merge (which I did by just taking your version of what was there).  Make further PRs against this branch.

Conflicts:
	doc/_templates/index.html