Add API section.

Very rough attempt, just to get this started. Addresses #109.

I'd like to hear your thoughts on how this looks, and adjust it based on feedback.

Sources of API section format inspiration:

- https://github.com/gittip/www.gittip.com#api
- http://developer.github.com/v3/

The following things should probably be added that I'm aware of:

- [ ] https://github.com/garyburd/gddo/issues/38#issuecomment-9146628 should probably be mentioned somewhere as well.
- [ ] API for refreshing a package.

It is up to you how public you want to make this API. Perhaps you might want to keep it in a separate API.markdown file, or something else. Just let me know.