[Core] Improved asset processing and URL rewriting in CSS
 * All asset processors now use a ProcessingContext which hold all needed information such as the asset to be processed and the current request
 * Removed FestAssert in favor of AssertJ and adapted all tests accordingly
 * Added a new url rewriter, making URL rewriting in CSS functional (lots of code comes from the Jawr project)
 * Improved comments/javadoc

Closes #22
Fixed job URL