Proposal for split testing

**What does this MR do?**
This PR is a proposal to change the way we currently run our tests. Because lots of the time travis will stop testing, because the limit of 50 minutes has passed.

By adding tags to tests (like I did on `models/assembla_service_spec.rb` and `features/gitlab_flavored_markdown_spec.rb`) we can split the tests up in smaller tasks. That way every task will take less time, and Travis will not time-out that often.

**Are there points in the code the reviewer needs to double check?**
If there is a possibility that we might skip a test this way (I dont think so, because of the `rake testing:other` task, but just to be sure, please take a look).

Please keep in mind that this is just a Prove of Concept, so the full implementation needs still to be done.

**Why was this MR needed?**
See *What does this MR do*