Revert to stable data formats as deployed on persona.org.

This removes a bunch of work on "new format" certificates and
assertions, which was designed to bring things into closer compliance
with the JWT spec but was ultimately never deployed.  It reduces
complexity and maintenance burden by remove formats that will not be
seen in the wild.
Merge pull request #103 from mozilla/rfk/remove-new-cert-format

Revert to old-style data formats as the only support format