Py-vapid

Switch to cryptography library

* Switch to python cryptography library
* BREAKING: Require `aud` parameter in VAPID declaration
* Reference proper specification
* dump the Application Server Key (used for restricted endpoint generation) with the `--applicationServerKey` param.

Python 3 compatible,
--json output
--gen key pair generation

DRAFT-02
See https://tools.ietf.org/html/draft-ietf-webpush-vapid-02


DRAFT-01
See: https://tools.ietf.org/html/draft-ietf-webpush-vapid-01