Introduce and add CSS generator SASS w/ library Compass.

Setup the development environment:
- *Gem*: adds a Gemfile definition ensuring Ruby dependencies
- *NPM*: adds `grunt-compass` using to call compass directly by Grunt
- *Grunt*: defines the compass task (dev/prod) and adds a corresponding watchers (dev only)
- *Git*: defines in .gitignore that the `.sass-cache` and the actual output `static/css` should be ignored.
- `*Travis CI*: define the correct SASS/Compass dependencies (see Gemfile!)

Migration of CSS -> SCSS:
- former `style.css` -> `_style.scss`, `_avgrund_override.scss`
- former `avgrund.css` -> `_avgrund.scss`
- compact index file `main.scss` with of three of them
- use as much Compass mixins as possible

- Change including CSS files in HTML files.