Added Flowdock integration support via a service.

Added test for the FlowdockService.
Use gitlab-flowdock-git-hook patched for dependency on gitlab-grit, instead of grit.

Make sure the flowdock-git-hook is loaded. Seems it is not autoloader, due to the gem/lib name mismatch.
Merge pull request #5216 from bladealslayer/feature/flowdock_integration

Added Flowdock integration support via a service.