Draft API method for merge MR

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>
Specify error codes for merge api

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>
Docs for merge api

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>
Add some tests for merge API

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>
More tests for merge api

Signed-off-by: Dmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>
Merge branch 'api-mr-merge' into 'master'

Accept merge request API

This MR adds new endpoint `PUT /projects/:id/merge_request/:merge_request_id/merge`. After this change you can merge branches using API.

Fixes internal issue #1166