Tidied up cache buster url generation

Used Assetic's worker support to replace the url modification done in the service.
Gives the flexibility to change how the cache buster url is created e.g. could use a hash of the content instead of the filemtime.  
The only thing I wasn't sure on, was the restriction to css and js files, it didn't seem to make any difference if it I removed it, and being able to cache buster images could be useful.