Switch to sphinx-better-theme

Includes #721 to avoid conflicts and so I could develop with livereload. It could easily be rebased back onto master. You should only need to look at `conf.py` and `style.css`.

* [Main page of demo](https://dl.dropboxusercontent.com/u/360865/mrjob_docs/index.html)
* [Representative narrative documentation](https://dl.dropboxusercontent.com/u/360865/mrjob_docs/guides/writing-mrjobs.html)
* [Representative API documentation](https://dl.dropboxusercontent.com/u/360865/mrjob_docs/job.html)

While doing the conversion and painstakingly checking every page, I found some ways I can improve `sphinx-better-theme`. I decided to just add them to mrjob's custom stylesheet for now rather than do a release that I'd have to go back and change if we found more improvements during this review. Including:

* Make the content area wider to better fit Python signatures
* Move the sidebar below the content if the screen is narrow (for mobile)
* Use the browser's default `monospace` font because `sphinx-better-theme`'s `tt` font doesn't support bold on OS X, and bolding really helps function signature legibility
* Make the links red (this will be mrjob-specific, not patched upstream; half the point of this theme is to be CSS-customizable)

BEFORE MERGING: figure out the situation with making `sphinx-better-theme` available to readthedocs.org. Might need to use a submodule or something.

**I seek your input** about whether or not we should hide one or both of the navigation bars and/or the sidebar and/or "Quick Search," or rearrange any other links/nav in any way. We are not limited in how we can ultimately structure the page.

Changes to be made to `sphinx-better-theme` to better serve mrjob that we can't make use of yet:

* Restructure main markup to make styling the header with color and a logo easier