Source maps

Merge master into the source map branch (fixes some syntax changes). We could rebase if you guys are au fait with that.

To make this actually useful there are about three parallelizable tasks:

1. Make sure this branch actually works inside rails (I didn't get it working, but probably user error).
2. Update the call to Uglify to use `compile_with_map`
3. Add `compile_with_map` to the coffee script gem.

I'm planning to spend a small amount of time working on this, but don't expect anything quickly.