Add syntax colors to diffs in email-on-push notifications

Closes Issue #6994 ... Diff syntax colors for email-on-push-service
Also closes feature request [5802256-add-color-to-diffs-in-email-on-push-service-to-html](http://feedback.gitlab.com/forums/176466-general/suggestions/5802256-add-color-to-diffs-in-email-on-push-service-to-htm)

![screenshot_mergereq](https://cloud.githubusercontent.com/assets/242848/4242251/d3f9ec34-39fb-11e4-9fc5-6813979598fe.png)

@oracle
@rodrigo-lima

I think the same applies here as the previous comment from @razer6, please move this to a helper