Implemented compute attach interfaces extension.

Attach interfaces API extension let to connect/disconnect running VMs to network ports. See http://developer.openstack.org/api-ref-compute-v2-ext.html#ext-os-interface. Sorry I'm have no time to cover it with unit tests, but I'm check API methods in sandbox project.
The name `IP` for this class is a little too abstract IMHO. I would rename this class to `FixedIP` to differentiate it from `FloatingIP`.