Initial GitLab Driver

This is a proof of concept, and mostly done to gather feedback on the
structure of the driver and to see if this is something that Composer
should include in core.

Tests are coming. I find the VcsDriver interface confusion, or rather the GitHub implementation, but this is a first stab at it.

Any feedback?

fixes #3241 
this method is useless. It is implemented in the abstract class VcsDriver