Use AssetHelper to prefix the baseurl

This ensures the requirejs baseurl setting respects the 'framework.templating.assets_base_urls' setting in the Symfony config.

Fixes hearsayit/HearsayRequireJSBundle#19

Enables scripts to be stored at a non-default path (e.g. not in the web root), and/or accessed via a different hostname (e.g. using a CDN).

See also: http://symfony.com/doc/2.1/reference/configuration/framework.html#assets-base-urls