Appveyor Upgrades - #1927

A couple of enhancements for appveyor:

• Commented out commands to get an RDP console on Appveyor's servers. These block, so we can't leave them in, but they can be useful to figure things out for people who don't have a windows machine they want to fire up.
• S3 deployment is only on the master branch. I'd like to make it just from python-pillow, but I don't see that in the docs. This prevents deployment failure stopping the test runs for everyone else.
• Added wheel builds as in Enable downloadable wheel builds on Appveyor #1815 by @techtonik, but with a specific option for it, uploads to s3, and less intrusive changes to the build script.