Support for new arguments in createMergeRequest method

[augi]

The createMergeRequest method could now support following arguments: target_project_id, labels, milestone_id, remove_source_branch. See the API description.

If you want then I can prepare a PR.

Btw. thanks for this great library!

[gmessner]

@augi
That would be great if you could submit a PR. I do have one request, could you create this as a new method? I try to avoid breaking anyone's existing code.