Add support for GitLab as a hosting service.
This adds support for the GitLab code hosting service. This is an open

source alternative to GitHub that supports Git repositories, issue

tracking, and more.
GitLab support takes advantage of plans. There are only two: "personal"

and "group".
The GitLab API, while mostly straight-forward, has some quirks. One of

those quirks essentially prevents us from supporting more than 100

repositories in an install (as lookups will fail). This hopefully won't

impact most people.

Unit tests pass.


Added a personal repository and a group repository. Both worked.


Posted code changes to both successfully.


Tested error conditions for:

Invalid username
Invalid password
Invalid hosting URL
Missing repository on personal and group plans.
File not found errors.