Replace compass build with grunt

Replace existing configuration for compass with a unified grunt config.
Three tasks are set up:

- `grunt`: Replacing `compass compile`,
- `grunt watch`: replacing `compass watch`,
- `grunt minify`: optimizing all scss files for production
Merge pull request #6 from fhemberger/v4.1.0

Replace compass build with grunt
First successful grunt build

Signed-off-by: Dirk Jesse <office@highresolution.info>
Updated grunt configuration to only create minified versions of core files

Signed-off-by: Dirk Jesse <office@highresolution.info>