Build and publish Windows image on AppVeyor

[ecstatic-morse]

This builds and publishes the Windows image added in #5 on AppVeyor. As with the Travis CI integration, DOCKER_{PASSWORD,USERNAME} must be set as environment variables.

[pietroalbini]

One small thing I'd like to get fixed. I'll setup environment variables soon.

[pietroalbini]

Configured the environment variables and the permissions, let's try this!

[ecstatic-morse]

Deployment failed.

I believe that DOCKER_USERNAME was not actually set, and --username ended up taking --password-stdin as an argument. I'll push some changes which explicitly check this.