Add appveyor.yml to test VS2015 builds.
Add appveyor.yml to test VS2015 builds.

Signed-off-by: Ralph Giles <65585f72bf62a11d2f4105aa30c20fe94efea46f@thaumas.net>

brion/ogg,ShiftMediaProject/ogg,xiph/ogg,xiph/ogg,ShiftMediaProject/ogg,mwgoldsmith/ogg,brion/ogg,mwgoldsmith/ogg