Extension: Keypairs

Implements the [Keypairs](http://developer.openstack.org/api-ref-compute-v2-ext.html#ext-os-keypairs) extension for OpenStack compute.

- [ ] Add the extensions to openstack/compute.
 - [ ] Delegates.
 - [ ] Acceptance tests.
 - [ ] Unit tests.
- [ ] List keypairs.
- [ ] Create or import keypair.
- [ ] Delete keypair.
- [ ] Show keypair.