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 :-)

Before recent changes in the project services you were limited in the properties available. Is that why you chose `subdomain` here? You can now specify any arbitrary property name as long as it's in the `props_accessor` method, too. Maybe consider changing this to be more meaningful.