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.
This came up in another PR I just commented on this week. Since we really want to know what fields are null, we should resist using `omitNullValues()`! Also, please change the case of the fields from snakey case to lower Camel case as they are defined in the class. `network_id` -> `networkId`