Added GitLab integration

This integration shows commit messages when pushing to [GitLab](https://www.gitlab.com).

The webhook url created by Hammock must be added to the webhook section on a repository.

Example:
![GitLab Integration](https://f.cloud.github.com/assets/1090754/2518535/40b647ea-b465-11e3-8e54-23a769b17b5b.png)