Add digital ocean provider

This is still work in progress, but I'm able to create, destroy, reboot, bootstrap, and deploy to servers on DigitalOcean.  This is the first provider other than EC2 to have been added to rubber.  Along the way I ran into a lot of rubber's heritage in being an EC2 only solution and cleaned some of that up as I went.  I'm not proud of some of the changes (using :respond_to? in several places), but without the benefit of a third provider, I can't really tell what the API should look like.  So, this is good enough for me for now and any changes would be internal anyway.

If you want to play with this, you do need to use a fog fork for the time being as the Digital Ocean provider hasn't been merged in yet.  The fork in question is:

https://github.com/rubiojr/fog/tree/digitalocean-compute

I'm probably going to punt on image creation for this pass.  We currently don't support EBS root volume image creation either, so I'd rather do them as a pair.