Refactor style_guide.md

[sffc]

We have added a lot of content to style_guide.md over the last two years, and some of it has evolved or become obsolete. We've done a fairly good job at keeping it up-to-date, but it would benefit from a birds-eye look on organization.

[Manishearth]

Also I think we need to have a TOC, it's kinda hard to find your way around