Optional usage of the Google Closure Compiler

Hi Samuel,

thank you for the plugin.

I have added the optional usage of the Google Closure Compiler as an alternative to the YUI Compressor using a <jsEngine>closure</jsEngine> configuration tag.

My closure-minified js file was a little bit smaller than the yui-minified one.

It's working for me although unit tests are missing, sorry about that.

Merry Christmas ;-)