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.
And in general, I find having to `cd` into the sass directory cumbersome, and this command is hard to remember. Maybe we could make a `manage.py` command to do this stuff? Could we tie sass generation into server startup (and automatic server restarting during development)?