Sass, bourbon, etc.

* Setup instructions which explicitly avoid inclusion of bourbon/neat in version control
* More or less copy-based the existing main.css into main.scss, then organized
* Removed many (but not all) references to pure

I no doubt missed a few styles but I migrated everything I ran into.
=) Yes, I just cut out `gulp` from `18f.gsa.gov` because of my own feelings about the complexity of generators and `[R|M|*]akefile` and stuff. But I'll be doing front-end dev on the project, and I'm hoping to not have to memorize `.:../css` ;)