Add an optional ID parameter

[sgarfinkel]

We have a situation where we would like multiple penpal connections between a parent and child iframe. However because the penpal handshaking protocol assumes that any handshake between the child and parent is the “correct” one, it discards the registered methods in previous handshake calls.

Instead, penpal should have an optional ID parameter that can be used to disambiguate these handshakes so that several connections between parent and child can exist at any given time.