Add tags to GitLab issues

It is just port from GitHub Plugin. Credits to @jessicard :)

I only made some tests with my GitLab 6.9.0.

```bash
coffee index.coffee --private_token="HIDDEN" --gitlab_url="HIDDEN" --project_id="Gitlab Test1" --labels="test @$%^&*()_+][}{\|;=-:>?<" - Pass
coffee index.coffee --private_token="HIDDEN" --gitlab_url="HIDDEN" --project_id="Gitlab Test1" --labels="test @$%^&*()_+][}{\|;=-:>?<, tag23456789" - Pass
coffee index.coffee --private_token="HIDDEN" --gitlab_url="HIDDEN" --project_id="Gitlab Test1" --labels="bugsnag, some tag, tag_two_with_dot ." - Pass
```

Resolve #42