Continuous integration with MSVC (using AppVeyor?)

Since I do not regularly use Windows or the MSVC compilers it would be great to have some continuous integration in place to automatically test cJSON on Windows with the Microsoft toolchain.

This would also provide a basis for adding compiler warning and something like -Werror for the Microsoft compiler.

If I have to do this by myself, I don't know how long it will take, so if anybody wants to step in and do that it would be great.

Note that I don't have permissions to add CI systems to this repository, so it would run on my fork FSMaxB/cJSON (like TravisCI)