AppVeyor CI support

This PR adds support of [AppVeyor CI](http://www.appveyor.com).

Similar to TeamCity there is a custom Visitor implementation which is auto-detected or enforced with `-appveyor` command line switch. `AppVeyorVisitor` uses [AppVeyor Build Worker API](http://www.appveyor.com/docs/build-agent-api) to communicate test results to AppVeyor real-time build console.