Add a haml-coffee-source gem.

This PR add generation of a `haml-coffee-source` gem, in the same maner `coffee-script-source`, `eco-source`, and other are generated.
It allows other gems to use an up to date version of the compiler's source code.
I've made a gem [ruby-haml-coffee](https://github.com/plietar/ruby-haml-coffee) which uses it and runs the compiler using ExecJS.

The gem is built using `rake gem`