Support more than 100 Gitlab hosted repos

Newer gitlab versions do provide pagination support via a header. See
https://github.com/gitlabhq/gitlabhq/commit/0678b8a4 Gitlab v6.8.0+

Add a new method that follows the "next" rel if present in the link header, and
use this to fetch all repositories.

Testing Done:
Tested on a gitlab 6.8.X deployment against a developer reviewboard.  Was able
to add a repository that's beyond the first 100 repos.

Reviewed at https://reviews.reviewboard.org/r/6037/