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.
There should be also tests for the `create` and `delete` methods, as long as tests that verify the fallbacks. The `list`, `get`and `delete`methods define a fallback, and there should be the corresponding tests to assert that jclouds returns the expected value.