Integration discussion: Gitlab

The discussion for the suggestion by @jtvn:

> I would personally like gitlab validation, as it is an open-source git hosting solution (offer free hosting at gitlab.com, and download for own setup at gitlab.org).

The fact, that Gitlab is not a central service provider but is a software which can be run anywhere doesn't make it that easy to add, since there is no base url, no fixed user base, etc.
It could be something like claiming websites, only less, since it is only a profile on custom websites.

Despite the named problems verifications could be done the same way as with github. There are snippets (like gists) and they use Markdown as well. There is even an [API](http://doc.gitlab.com/ce/api/project_snippets.html) to ease the crawling.

Review the GitLab validation discussion and the linked GitLab project-snippets API documentation, then compare it with the existing GitHub verification flow. Define how custom GitLab instances and profiles should be identified; done means a decided integration design and documented verification behavior.

Written by the indexing model from the issue text.