Support Sprockets use outside of Rails.

This change registers the CSS and (most importantly) JavaScript paths directly with Sprockets when Rails is not loaded.

This makes Foundation work with Rack-based applications that use Sprockets but don't use Rails. I tested it with a middleman site I'm working on.

I'm not familiar with the foundation codebase, and I'll be happy to address any feedback on style / structure / tests. (I only did manual testing)

Thank you very much for foundation!