[CI] Add appveyor.yml configuration file to support Windows build

Adds a configuration file to facilitate build of Mono with Cygwin
on Appveyor Windows-based build workers.

Dependencies are installed on a clean Appveyor build image for
each build and the result of the 'make install' step is stored
as a build output 'artifact' for further testing or deployment.

Signed-off-by: Alex J Lennon <ajlennon@dynamicdevices.co.uk>