Add configuration for AppVeyor, a Windows CI service.
Add configuration for AppVeyor, a Windows CI service.

This configuration will build and test the core of Racket on every
commit. The time constraints are such that only a small portion of
Racket can be built and tested.

mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket,mafagafogigante/racket