provision/juju: implement Register method on ELBManager

Related to #244.
api, service: update date in license headers
provision/juju: implement Deregister on ELBManager

It just deregister instances using Amazon Web Services API.

One step closer to getting #244 done :-)
provision/juju: implement Addr method on ELBManager

ELBManager finally satisfies provisioner.LBManager functionally.

Related to #244.
provision/juju: unexport LoadBalancer

Related to #244.