issuer in OBI assertion should be the issuer organisation, not the user

[askvictor]

At least that's my reading of the spec: https://github.com/mozilla/openbadges/wiki/Assertions

EDIT: in fact, the Issuer part of the assertion in django-badger is outdated; url (rather than location) and name are mandatory; email replaces contact.

[lmorchard]

This should be addressed in this pull request: #195