Sass: refactor and make more themeable

  - sass: put partials in lib/{.,mixins}
  - document and make themeable
  - add example themes: 'flatly' and 'darkly' from bootswatch

Squashed commit of the following:

commit 5af8d680ff4cdf4ff98beae6e8e156b6045a206b
Author: Max F. Albrecht <1@178.is>
Date:   Sat Dec 13 19:11:21 2014 +0100