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.
I may touch this up later -- if someone actually develops with Ruby on Linux (as opposed to just needing it around to meet dependencies), `apt-get` is not the path to use, the packaging is poor for day-to-day use. They'd want to use [`rbenv`](https://github.com/sstephenson/rbenv) or something like that. But to "just get it installed" that works just fine.