Added AppVeyor CI script

[AppVeyor](http://www.appveyor.com) is a continuous integration service that is free for open source projects

The build steps are configured in appveyor.yml, it'll be recognized as soon as you login to AppVeyor.
See here how it looks: https://ci.appveyor.com/project/akoeplinger/monodebugger

I also had to make a few small fixes to the csproj files to make it work under Release config.