Utilize pagination headers

Takes a swing at issue #51.  Before enhancing, I initially added monkey-patches to accommodate my migration of repos from github to gitlab, then decided to make it a Real Change, then noticed #51 was already created; I added some tests for the "/users endpoint", made them pass, and replaced the rest of the endpoints that needed "paginate".  I built a local version of the gem for my testing, threw away my monkey-patches and integrated the paginating version of the Gitlab gem back into my migration scripts.  (The "/projects" endpoint also got a workout at this point).  The existing tests still pass.

NOTE: These changes have NOT been tested via direct invocation of the Gitlab gem's command-line interface, but only via unit tests and calls via inclusion into my scripts).