Add version badge to Docs

Figured how to show the badge with the current theme. 

The `current version` is taken from `release` variable,
but the previous versions are hardcoded, need to be
updated manually every big release for now.

![screen shot 2014-12-02 at 8 26 42 pm](https://cloud.githubusercontent.com/assets/1294734/5275864/a19828f4-7a61-11e4-99b6-7053c0edd41a.png)