Add initial AppVeyor config

[paultcochrane]

AppVeyor is a continuous integration service which is free for Open Source projects which allows one to build and test software on the Windows platform. This PR includes an initial configuration file for this service; the dist builds and tests successfully with this configuration.

If you have any questions or comments concerning this PR, please just contact me and I'll update and resubmit it as needed.

[hoytech]

I've never used appveyor before, and I don't use windows at all, but if you find it helpful I have no problem adding. Thanks!

[paultcochrane]

I also others will find it helpful :-) Actually, I don't use Windows either, however many people do, and every time i add the Travis config I always think it can't hurt to add the AppVeyor config either. It's sometimes amazing the problems one finds on a another platform that weren't obvious in the dev environment. Perhaps by using AppVeyor one can avoid CPAN Testers failures. Ya never know :-)