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
Merge branch 'upstream/master'

Conflicts:
	Configuration/ConfigurationBuilder.php
Correct Assetic base URL was never being used, fixes bug introduced by 51748ca
Correctly handle case where 'templating.helper.assets' service is not defined
Fix tests involving assets base URL
Code style
Merge pull request #55 from sgarner/fix_baseurl_bug

Fix bug where asset base URL was not being used again