Updates to the Grails plugin

I bumped the version to the latest version of wro4j, updated the README.md, and did some other minor cleanups.

This should facilitate publishing to Grails Central: https://code.google.com/p/wro4j/issues/detail?id=707
And allows a new release: https://code.google.com/p/wro4j/issues/detail?id=708