Fix #158: Updates to use latest OBI JSON specs

* Update OBI serialization in Badge and Award to match latest spec:
    https://github.com/mozilla/openbadges-specification/blob/master/Assertion/latest.md

* {badge,award}.get_absolute_url has a format='json' option

* New URL at issuer.json that serves up the BADGER_SITE_ISSUER data

* Updated BADGER_SITE_ISSUER setting
    * See test_settings.py for an example

* Tests