Add support for obtaining version info

[Duncan McGreggor]

The Rackspace CloudServers API supports obtaining version info via simple GET against servers.api.rackspacecloud.com. We need to support this.

The best option is probably a dedicated query object, similar to what was done with AuthenticatingQuery (e.g., VersionInfoQuery). A method on the client would use and instance of this custom query object instead of self.query_factory, which is used for API calls. This will allow a custom endpoint without conflicting with the endpoint in use my self.query_factory.