Add support for IE11 via iframe hack.
Add support for IE11 via iframe hack.

- IE11 does not support using postMessage when using cross-domain windows,
  only cross-domain iframes. This hack loads the credential agent (auth.io)
  in an iframe instead of in a separate window when IE is detected.

Spec-Ops/credentials-polyfill,Spec-Ops/credentials-polyfill,digitalbazaar/credentials-polyfill,digitalbazaar/credentials-polyfill