please review credman PR#100 and webauthn PR#498.. (fixup algs contd 3)

..when you guys & other webauthn/webappsec denizens are able :)

as noted in
https://lists.w3.org/Archives/Public/public-webauthn/2017Sep/0102.html:

I have made further changes to accommodate @mikewest s initial feedback, 
which he gave over in the associated credman PR: 
https://github.com/w3c/webappsec-credential-management/pull/100#pullrequestreview-58690526

Depending on what we decide over there in credman wrt whether we add an 
`origin` member to each of [CredentialRequestOptions 
Dictionary](https://w3c.github.io/webappsec-credential-management/#credentialrequestoptions-dictionary) 
and [CredentialCreationOptions 
Dictionary](https://w3c.github.io/webappsec-credential-management/#credentialcreationoptions-dictionary), 
there may be further commits to add to this PR. C.f.: my comment: 
https://github.com/w3c/webappsec-credential-management/pull/100#issuecomment-327968338

thanks,

=JeffH