[w3c/webauthn] 835e72: Clean up exposition

  Branch: refs/heads/vgb-u2f-attestation
  Home:   https://github.com/w3c/webauthn
  Commit: 835e72cf74937b0c047e924b0ba067f0475cc8eb
      https://github.com/w3c/webauthn/commit/835e72cf74937b0c047e924b0ba067f0475cc8eb
  Author: Vijay Bharadwaj <vijaybh@users.noreply.github.com>
  Date:   2017-02-11 (Sat, 11 Feb 2017)

  Changed paths:
    M index.bs

  Log Message:
  -----------
  Clean up exposition

Separate out verification of assertion and attestation signatures,
removing redundant steps. Broke up signature format sectiona and moved
the pieces to the appropriate places.