Partial fix for Tfs sdk 12.0.2

Partial fix for #234. Seems to be working with visualstudio.com with alternate credentials, although I do not have a working build task to try it out on yet. It gets the revision id and commit message in any case.

This code does not use the domain variable. That can added easily by prepending "DOMAIN\" to the username in UsernamePasswordCredentials, according to the javadocs shipped with the SDK. The [Jenkins TFS plugin](https://github.com/jenkinsci/tfs-plugin/blob/master/src/main/java/hudson/plugins/tfs/model/Server.java#L39) supports NTLM authentication by explicitly using DefaultNTCredentials, which has been deprecated in favor of using UsernamePasswordCredentials with the username formatted as "DOMAIN\username" or "username@domain".

Also, no work has been done to support unique workspaces mentioned by @Christopher1065.
can you please add a test for this?