[WIP] TeamCity meta runner

Happy to submit this to https://github.com/JetBrains/meta-runner-power-pack if you think that's a more appropriate place. I just figure it's easier for you guys to keep it up to date in this repository?

I have only run the .ps1 locally so far (see the test.ps1), but I'm about to try it out on my TeamCity server.

Two things I'm not sure about (I suspect they can both wait for later since they are potentially not a big deal for now):

1. You can't include spaces in the `execArgs` or `projArgs`
2. Once this gets a gitversion installed on the server from Chocolatey it will never try and update it - I could include a call to `cinst` to update it if necessary every run, but that will add at least 1-2s (if not more) to every build run due to Chocolatey's slowness. That's why I'm checking the filesystem for chocolatey rather than invoking a Chocolatey command to see if it already exists.