assets plugin - initial commit - all tests pass

It would be better if `Content-Type` was made one of default headers used by the assets plugin, so you could eliminate the `merge` and just use `merge!(scope.assets_opts[:headers])`.  That would require splitting the headers hash into two hashes (both based on the `:headers` option), but you could also do the `Rack::Mime.mime_type` call when loading the plugin, so that you wouldn't need to call it every time.