support for groovy rules in intellij project generation

Created by: mread

Not quite right yet but for your consideration.

This pull request supports groovy_library and groovy_test rules as intellij source folders.

Outstanding issues:

1. what additional unit or integration tests do you want to see?
2. I'm not sure what addCompiledShadowIfNeeded() does but I'm not calling it, should I?
3. there's an unnecessary <exclude> and a packagePrefix being created in the .iml, any suggestions on the best place to look for a fix?