Preliminary updates for new BrowserID formats

In-progress work to update PyBrowserID for latest BrowserID protocol and format updates.  This should cover the data format changes, yet-to-do are updates to the discovery protocol and the notion of "trustedIssuers" provided by the new nodejs verifier.

I'd like to get cracking on this so that I can use PyBrowserID's testing infrastructure for tokenserver tests, which now use some new features e.g. "idpClaims".

@ametaireau and @callahad any f? on this would be appreciated if you've got a few moments.