Fixes #16.

Add background back, along with a box shadow, for that nice
Twitter-like blur out of the covered text.

Keep broad resets namespaced and out of common.scss.

It's necessary to bring the heatmap back inside the margins so that
it's visible, which requires some retooling of z stacking to get event
bubbling to behave.

Add threaded message reply support
- Significant refactoring of templates to reduce context sensitivity.
  For example, the editor can not be rendered inline inside a thread.
- Refactor app.scss for a cleaner threading markup.
- Add jwz.js conversation threading (thanks @maxogden) and underscore.js
  (needed by jwz 😡 @maxogden)
- Lazy-diff: switched yepnope.js LICENSE to WTFPL rather than relicensing
  under MIT. Even if we don't go the WTF route for the whole project, it's
  fun to let that license ride along in our documentation.