[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>
Fix for ObjectDiposedException in DispatcherCB class

https://bugzilla.xamarin.com/show_bug.cgi?id=23700
Merge pull request #1369 from DynamicDevices/ajl/appveyor

[CI] Add appveyor.yml configuration file to support Windows build
Merge pull request #1373 from michaldobrodenka/master

Fix for random ObjectDiposedException in DispatcherCB class when connecting