dropbox-go: appveyor build & test script

This script was copied from cookieo9/go-misc and modified.

Fingers crossed....
Added AppVeyor status badge to README.md
Merge pull request #5 from cookieo9/appveyor

Merge AppVeyor Support
Merge branch 'master' into flattened

This should add the appveyor support, but it does need to be adjusted
for the new source layout.
Updated AppVeyor support for flattened repository