Add appveyor support

Hi!

I've added [appveyor](http://www.appveyor.com/) support to the test reporter. AppVeyor is a Windows CI environment. This is the documentation for the environment variables: http://www.appveyor.com/docs/environment-variables.

While testing the new feature against my gem, I discovered a windows specific bug which I've fixed as well. [This](https://ci.appveyor.com/project/deivid-rodriguez/byebug/build/46) is an example of a build experiencing the bug and [this](https://ci.appveyor.com/project/deivid-rodriguez/byebug/build/47) is another one with the bug fixed and the coverage data properly sent. You can look for the string `Committed at` in those logs to see the bug.

Cheers!!