Extension: Keypairs

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