Provide Public Key Registration for VAPID

[jrconlin]

Continuing the work of #325, clients can provide a Public Key as part of the register() call.

Incoming subscription update requests (Crypto-Key Header; p256ecdsa component) must match the registered key.
Requests that do not match shall be rejected with a 401.
Requests that use the same public key for p256ecdsa and dh components shall be rejected with a 400.

It has been suggested that a hash of the public key be stored in the endpoint, which may require endpoint versioning.

[jrconlin]

closing as duplicate of #326