Support for signed badges

[davelester]

Signed badges use the Backpack javascript issuer API to pass a signature rather than a URL. The JSON representation of the badge assertion should be used as the JWS payload. More info: https://github.com/mozilla/openbadges/wiki/Assertions

[davelester]

Not currently on the roadmap, but anyone wants to pick up this issue and run with it that would rock.