support for s3 storage class

While not in the issue backlog, I added support for controlling on a per-project basis what s3 storage class is used. I wanted to get my feet wet contributing, and at work I make heavy use of this plugin for artifact back-ups; should save us some monies.

I tried to change as little as possible, even though I was sorely tempted to perform some mechanical refactors. Let me know what more I can do to get this accepted?