Initial parent-iframe handshake

[iparamonau]

See general JS events/methods description by @bdougherty in #1.

For general discovery, there is an idea to add initial handshake, where iFrame will post a Message to the parent with the list of events/functions it supports and that parent can use.

Anyone would like to share of thoughts how to do it?

Also, take a look at the current spec where it says about method register with windowId.
Let's change it for general handshake mechanism, if we need one.

At the very lease, ID certainly makes it easy to communicate between parent and iFrame. We might, for example, also include format=JSON or =string communication on initial handshake, so parent knows in which format to communicate messages to iFrame. (@panzi please join)