Removing the Rails Controller and refactor into middleware

By using Rack middleware, we would allow easier integration of Jammit into non-Rails projects and addresses https://github.com/documentcloud/jammit/issues/161

The basic approach here is using a Rack::Request and extract path_info from there, package the assets with Jammit, and wrap it inot Rack::Response.

I was looking into Rack::Sprockets and Warden for some inspiration on the topic (incl. use of RSpec).

It would be great to hear your feedback what you think.

My first thought was also to rename the Controller into Manager or Packager, to avoid confusion with a Rails or Backbone controller.