Add Gitlab to Teatro.io

Hi! I want to offer you our service for automatic staging per each opened pull request. 

Now we run our service [Teatro.io](http://teatro.io) in beta testing mode. But we'd already check that Gitlab runs in minimal configuration, but required some settings that are subject of this Pull Request. 

This is how url for staging looks like (for my Gitlab fork): http://master.b8fb4f0c5117456ba90b7d95d54c7ddb.ttrcloud.com. Such link will be put into each pull request after opening (as comment from @TeatroIO user).

It's free for open source projects, of course. All what you need is to receive invitation (you should be owner or admin of this repo) and add repo to our service. Please provide me email and I send you invitation.

If you have any questions I'll be glad to answer by email/skype (mikdiet@gmail.com / empiromd)