Support uploading test results XML to AppVeyor

[robertcoltheart]

AppVeyor supports displaying test results in the UI in a user-friendly way. However it only seems to do this when AppVeyor is in charge of running the tests, via configuration of the appveyor.yml file.

If we run tests using Cake, it would be nice to be able to have AppVeyor recognise the test results and display them in its UI.

To do that, it seems the API route is the accepted way of doing this: http://www.appveyor.com/docs/running-tests#uploading-xml-test-results

Adding a method to IAppVeyorProvider to do that seems like the best approach here, no?