Basic Teamcity agent - only supports guest authentication, default branch

Basic support for the TeamCity CI server (https://www.jetbrains.com/teamcity/) in a similar style to the Jenkins agent check.

Unfortunately I'm not very well-versed in Python testing techniques so I wasn't able to write any tests here. Help with that would certainly be appreciated.
Also  before parsing it as JSON and seeing what's there, you should check the return code from requests to see if the request succeeded.