Refactor all the things

- Remove Guard completely, and replace with a simple Listen implementation. This still needs some work when it comes to removing and selectively re-copying certain files.
- Sprockets now manages images and fonts as well. I'm actually toying with the idea of re-adding ERB support to the php templates, and adding in Sprockets asset url helpers to the context. Also, the sprockets env now lives on the project instead of the builder, so you can more easily introspect registered assets.


cc @gf3 & @alexblackie