Automatic detection of TeamCity coming in v3.0.7.

[Jeff Brown]

I just finished adding some code that will allow the TeamCity integration
extension to be automatically registered whenever the TEAMCITY_VERSION
environment variable is detected.

As a result in v3.0.7 you will no longer be required to specify the TeamCity
extension as a parameter to Gallio.Echo or the Gallio build tasks.

This is just one of the many enhancements made possible with the new Gallio
plugin model in v3.0.7.

Jeff.