Add alert if the tracking branch is master for Discourse

This PR adds alert if the branch for Discourse is master.

![screen shot 2014-06-29 at 01 12 26](https://cloud.githubusercontent.com/assets/5732281/3421731/334d350e-fefd-11e3-81ac-4a9d9c835a86.png)

The alert message needs to be improved, I'll create a how-to on meta to explain how to change tracking branch.

This is a WIP.