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

### Testing:
1. Run `npm install` first to grab build dependencies
2. Simply running `grunt` or `grunt minify` should work, if grunt cannot be started properly, try `npm install -g grunt-cli` (which installs the grunt task runner as a global dependency). In this case, the additional step has to be added to the Readme file.
3. **Important: The 'yaml' folder is erased completely before each build, so don't store any additional files there.**