Extension: Disk Config

Implementing the [disk-config extension](http://developer.openstack.org/api-ref-compute-v2-ext.html#ext-os-disk-config-servers) for OpenStack and Rackspace compute.
I'm inclined to agree. The logic is that you can perform one `Get` call, stash the `GetResult`, and use different `Extract` functions to pull data relevant to different extensions from it without having to make additional calls.

I think the extension mechanisms we're using need a little work, still - it's awkward to compose extensions and it's all a little too manual right now. I'm not sure what I'd rather see, though, and I think it'll need to wait for post-release. I'll try to articulate my thoughts about what I'd like to accomplish in a discussion issue, possibly after these next two weeks are over and we have 1.0 shipped :smile: