Documentation updates

Here are a variety of changes that clean up and improve the documentation. My main aim was to remove the `plot_directive` and use matplotlib's copy directly. But then I found a couple of other things, and, well, here we are...

A short summary though:
 * Fix a lot of Sphinx warnings/errors for missing newlines.

These ones are moved to #1356:
 * Make use of `plot_directive` directly from matplotlib.
 * Clean up Sphinx extensions so they're PEP8 compliant.
 * Remove hard-coded front-page images so that they're _actually_ random.
 * Add missing and remove extraneous toctrees to quiet Sphinx.
 * Categorize example plots in broad divisions. Mainly because I rebased the `gen_gallery` extension to matplotlib 1.2's version, and it offered that functionality. The categories I chose are pretty arbitrary, and I'm open to suggestions.

Note: I tried to make the changes compatible with matplotlib 1.2.0, since that's the minimum version required in the docs. However, I haven't really tested with it because whenever I try to build docs in conda, it just hangs.