Added support for appveyor CI server

Added support for http://www.appveyor.com/ CI server. This still (of course) needs COVERALLS_REPO_TOKEN to be set (I set it as a secure env var).

I have tested it works with https://coveralls.io/r/ianheggie/net-ping2 - the only problem is coveralls reports the author of the commits as 'Ian Heggie' rather than ianheggie, and doesn't indicate I am a github user, not provides a link. I am unsure how to pass the value.

I test using both travis and appveyor - is there a way to have history by date/time rather than job number? Since this puts the last appveyor result on the last page, even though its close in date/time to the first travis entry on page 1.

https://coveralls.io/r/ianheggie/net-ping2?page=3