simple teamcity support

FR http://feedback.gitlab.com/forums/176466-general/suggestions/4139027-teamcity-integration

Simple Teamcity integration for MergeRequest build status and link on a build.

Algorithm: get status of the last build in Teamcity for concrete Teamcity.BuildConfiguration_ID and MergeRequest.SourceBranch and MergeRequest.Commits.Last

Algorithm is very simple but it is working for us since 2014/07/15 and everybody happy :-)

Adding the `branch` param is going to break every service, not just CI services, if the method signature isn't changed on every single one. Is there any other way to obtain `branch` without adding a new param?