Rackspace block storage

This PR adds support for Rackspace's implementation of Cinder; covers #218 task
I've been doing this by storing the response body in a `const` and making a `HandleXyzSuccessfully(t *testing.T)` function that returns it. There are a *few* cases where I add a "response" parameter to the Handle function to let me vary the response JSON but keep the status and headers the same in the test for a delegated call. This is just fine though. I like the "MockXyzResponse" names better, actually.