Remove Windows 64 bit downloads.

32 bit app works fine on 64 bit, and it makes auto update much easier to
deal with. Also, people don't have to worry about which one to download
anymore.
Upload appcast items. Upload a built version of master

With this change, master is alway built. That means you can download the
latest build. This is to make testing releases easier. Note that master
builds do not auto update.