Add General API navigation section.

- Adds a General navigation section
- Moves old top-level Gitignore and Markdown navigation sections into General
- Moves Meta and Rate Limit navigation sections into General
- Adds the newly documented Emojis section into General

All existing URLs remain unchanged. The /v3/general URL has been added as a summary page for the General navigation section.

![screen shot 2013-09-11 at 17 18 47](https://f.cloud.github.com/assets/65057/1123854/7b422c9e-1af5-11e3-8096-c15134fead09.png)

For your consideration. :v: