New builder

Here we go again, 

**:sun_with_face: pros:**
- pluggable processors (see `builder_processors.rb`), should be really easy to add more
- cleaner object design and less code
- support for file preloading (this means making files available for dynamic requires)

**:suspect: cons:**
- seems to be slower (but I think it's nothing we can't identify and fix)
- had to remove most specs since they were too coupled to the old builder
- source maps are still stubbed (but they'll come, using https://github.com/maccman/sourcemap)

**:shipit: I think we should merge because:**
- :trollface: yolo  
- :put_litter_in_its_place: it's better OOD 
- :seat: we can fix stuff (i.e. "cons") on master