use grunt-contrib-compass for theme compilation

This is a backwards compatible switch to using grunt-contrib-compass to compile theme assets, instead of building the command in a shell task. Ideally the options object for grunt-contrib-compass could be defined directly in the Gruntconfig.json (and merged with the defaults), which would allow full control over the theme tasks from config. That may require some breaking changes to the config though, or some extra work to support the current config structure.