Migrate to python-gitlab for backend/http requests

This will automatically solve at least the following issues:

• Marge fails too hard on network errors (#314 - closed)
• Pipeline.pipelines_by_branch() should use api.c... (#170)
• 504 GATEWAY_TIMEOUT fails instead of wait and r... (#144 - closed)
• "Too Many Requests" (#126 - closed)

And probably more. /cc @okias @JohnVillalovos just FYI :)