Use maven for the grails plugin, including build and release

This change makes the Grails plugin into a maven module, so the typical maven commands and workflow, including release as described at https://code.google.com/p/wro4j/wiki/ReleaseSteps , work as usual. This means no Grails knowledge is necessary, no special process is required for build or release, and the Grails plugins can always be released in step with wro releases with no additional effort (and if a wro change breaks the Grails plugin, you'll know immediately).

The build is currently failing to an unrelated problem in wro4j-core, but the grails plugin does build successfully and I have tested it in my own project.