Add flag to enable/disbale build only tabbed refs

Hi guys.

We started using gitlab + gitlab ci recently and here's a feature we really lack of: sometimes developers push their feature branches to remote to share code they're working on together or event to share code between work stations of single developer with failing specs. It's ok in our case, but we don't want to build it and get emails, etc. So it would be nice to have ability run CI only for specified branches (for example master branch and release branches) and ignore the rest. I added flag to enable running CI only against tabbed branches.