Fixes #375 - Use yuicompressor as optional Maven dependendency
Issue #375 - Add javadoc how to use it
Issue #375 - Add javadoc to YuiCssCompressor
Merge pull request #378 from l0rdn1kk0n/375-YUI-compressor

use yui compressor maven dependency instead of copy
Merge branch 'master' into wicket-7.0.0-M1