CSS nomenclature

This is an early PR to gather some feedback on what else folks would like to see in the Code Guide. I often run into problems when describing the content of a CSS file and until a couple years ago I didn't know most of these terms by heart either. Documenting the most common ones with examples seems like a good idea, but it's a lot of new content.

Here's how it looks currently with just a few items stubbed out:

![screen shot 2014-03-05 at 1 30 38 pm](https://f.cloud.github.com/assets/98681/2338786/767c46b2-a4ad-11e3-87f3-dcd4108c579b.png)

Some things I'd like feedback on:

* Should nomenclature be it's own section, split from the main HTML and CSS ones? Perhaps as a glossary?
* What level of detail is too much? Should we do into every type of selector?
* Is there another format that would serve this better?

<3