Added Atlassian Stash integration

This integration shows commit messages when pushing to [Atlassian Stash](https://www.atlassian.com/software/stash).

The webhook add-on is required on the stash instance and is available for free from the add-on marketplace in Stash. 

Example:
![Stash Integration](https://f.cloud.github.com/assets/1090754/2518434/342afe68-b464-11e3-8948-694a4eefb737.png)