Detach servers (remove from group without removing in Nova)

This adds the "detach" feature to the API.

- I used the term `purge` to mean "also kill the machine in Nova". I started with `delete`, but it seemed really awkward to have a DELETE request that is documented as deleting a server and then has a `delete` query argument...
- I spent a good chunk of time cleaning up the tests and fixing docstrings. It would be useful to give those a close (review) inspection to make sure I didn't screw it up.
> I guess I misunderstood what you intended to say after all, and "add delete=True to the REST API" wasn't very unambiguous. Sorry :(

No no. Please don't apologize. Its just a matter of misunderstanding. I also should've been clearer. We've thought about having 2 arguments to DELETE endpoint but decide to have it separate endpoint to make it easier for the user to grasp since we will have to document matrix of what each combinations mean whereas with DETACH it is obvious because of its name.