Cleanup webassets compiling

* Compiled assets now go together in assets/
* Javascript bundle is split into two parts: Pre-Compiled (for libs)
  and application javascripts. rjsmin was trying to minify already-
  minified assets and it was causing errors. All of the javascript
  will still be concatenated together in assets/min.js.
* Minified jquery-ui.js (using uglifyjs)