Adds `source/refills/javascripts/*.coffee`

https://trello.com/c/Vx9B2YMV

Defines a coffee script compile rake task to be run before `rake
{spec,build,publish}`. Coffee source files will be compiled to javascript.

This will eventually allow us to post the `.coffee` source files, in addition to
the javascript source files.
I am definitely *least* confident about changing this file, since the JS itself is rather confusing. I'd love for someone to look this over.