Initial work on asset compilation as an engine.

This is designed to allow asset compilation of Katello assets. These assets are generated in `katello/public/assets` and made to be served up by the webserver when accessing a Katello page. To test this:

Disable Katello in the local gemfile and compile Foreman assets:

    rake assets:precompile

Re-enable Katello gem and compile Katello assets from Foreman checkout:

    rake assets:precompile:katello

Serve static assets with Rails server by opening `foreman/config/application.rb` and adding the following among the other config:

    config.middleware.use ::ActionDispatch::Static, "#{Katello::Engine.root}/public"

Start Rails server in production:

    rails s -e production