Add ability to register multiple events with on.

Pulling the discussion out of #1206 about registering multiple events in a single call to `on`.

Tests include ones missing for `on`/`once`/`un`/`unByKey` that I think should probably be in there regardless of whether this gets merged or not.  If changing `on` is ultimately undesirable, I'll add the tests in a separate PR.

/cc @elemoine