[w3c/webauthn] b2ecda: Many attestation and signature cleanups

  Branch: refs/heads/vgb-u2f-attestation
  Home:   https://github.com/w3c/webauthn
  Commit: b2ecda2430d6cc50f4e848cfa337f6efe2fbc63b
      https://github.com/w3c/webauthn/commit/b2ecda2430d6cc50f4e848cfa337f6efe2fbc63b
  Author: Vijay Bharadwaj <vijaybh@users.noreply.github.com>
  Date:   2017-01-10 (Tue, 10 Jan 2017)

  Changed paths:
    M index.bs

  Log Message:
  -----------
  Many attestation and signature cleanups

Refactor the attestation section to clean up exposition. Separated out
signature verification (per format) from trust chaining (done at higher
layer).

Created a separate section for specifying key RP operations. Fixes #88.

RP registration section defines binding of credentials to user accounts.
Fixes #13.

RP registration section also defines options in case of registering the
same credential to different users. Fixes #12.

Cleans up and completes defining the process for verifying assertions,
which had already been largely done by @rlin1. Fixes #102.

Completes drawing the distinction between assertion and attestation
certificates. Fixes #118.

Replace "client platform" with "client" in signature format section to
avoid confusion. Fixes #209.