use iframePhone for cross-domain embedding in CODAP

Additionally, removes extraneous iframePhone listener removal from the ParentMessageAPI singleton. This is necessary to allow dgExporter to add its own listeners (via IframePhoneRPCEndpoint)

Note that this removal was extraneous anyway; the behavior of iframePhone.addListener is to replace the listener with the given name, so there was no danger of creating stale listeners if a ParentMessageAPI is created twice for some reason. Which I don’t think it ever is, anyway.
Merge pull request #70 from concord-consortium/codap-cross-domain

use iframePhone for cross-domain embedding in CODAP