Fix #158: Update JSON formats to latest OBI spec

* Update OBI serialization in Badge and Award to match the [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

* Tests