Improve SASS compile speed

Following the steps described in this article (http://blog.55minutes.com/2013/01/lightning-fast-sass-reloading-in-rails-32/) I was able to reduce the SASS compile time quite significantly:

*Before*
Change to application.scss padding value: 3.67s, 3.67s, 3.63s
Change to customizations.scss color value: 2.22s, 2.25s, 2.22s

*After*
Change to main.scss (=application.scss) padding value: 2.31s, 2.29s, 2.33s
Change to customizations.scss color value: 1.40s, 1.35s, 1.37s

The main improvement points:
* `@import` only needed Compass mixins, not the whole framework (i.e. remove `@import "compass")
* Create index.scss file, which utilizes Sprocket
* Use Sprocket `//= require ` instead of `@import` when ever possible

If we continue the process of splitting the massive application.scss file (which is nowadays called main.scss) we might get even better results in the future