Copy assets from React.js refactor branch.
Big asset-pipeline spike!

* Removed unneeded cruft from rebar.config.
* Added minification and JSX transform tools from NPM.
* Normalized names of vendored JS and CSS, removing pre-minified
  versions.
* Wrote the super-magical assets.mk which builds all of the files as
  necessary, learning a lot about Makefiles along the way.