Idp rework

Includes: 
* references to base64 RFC
* IDN RFC, in particular the rules for domain comparison
* more formal formulation for the .well-known URI (w/ reference)
* Note about inclusion of '/' and '\' in IdP protocol
* Started the process of removing product-specific stuff (facebook)
* Added WebMessaging reference for MessageChannel

This depends on all the other stuff I've sent you.  I'm still reformatting stuff as I go.  If that's a problem, I'll try to stop.