Compress Javascript and CSS files to reduce download times

XWIKI-1701 doesn't really work, since the yuicompressor plugin is executed after the war was created. The only effect is that it stops the build if JS errors are found in the files.

Another problem is that even if the compression was executed at an earlier phase, the version of the war plugin currently in use overrides the modified files with the clean sources (see http://jira.codehaus.org/browse/MWAR-60 ).