Make docs versioning more apparent

* Show note if docs are for a dev version, link to stable version
* Use 'versionadded' in the sole new feature of 0.4.1, `MRJob.SORT_VALUES`

![screenshot](http://dl.dropbox.com/u/360865/Screenshots/5v.png)

![screenshot](http://dl.dropbox.com/u/360865/Screenshots/5w.png)