Update OBI JSON generation for new assertion spec

[lmorchard]

There's a new JSON assertion spec in the works. Make changes to support this new format.

[lmorchard]

See also: this issue in badges.mozilla.org asking for a tweak to JSON feeds to use the OBI displayer format

[sunshineo]

Real spec link. https://github.com/mozilla/openbadges/wiki/Assertions#wiki-verification-object This should be easy. I'll try.

[sunshineo]

Not easy. Missing the Badge Class endpoint.

[lmorchard]

This should be addressed in #195