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.
Hey Leo. Thanks for all the feedback on this! I'll get things fixed up.

I specifically made server an init_config parameter as I figured most people would use this check on a single CI server (TeamCity isn't typically clustered or anything), but for multiple build configs on said server. If that doesn't make sense, I can move it to an instance parameter for sure.