DAR-826: fluid width background image styles implementation

__Please don't merge until code review is complete__

https://wikia-inc.atlassian.net/browse/DAR-268

This pull request introduces a solution for background images with a fluid layout. It was designed to use the wiki's current background image. The basic premise is that it will split the image in half and align it properly inside the viewport as the layout is growing and shrinking.

This changeset introduces the [Bourbon](http://bourbon.io) Sass library as our current linear-gradient mixin is outdated and doesn't support what I needed to do. We plan on using this library for most things in the future when all of the responsive layout stuff is refactored when it launches globally, so it won't be a single use case here.