Support type-less listener registration.

[nbubna]

For cross-cutting concerns like logging and debugging, it would be convenient to support registering listeners by category (or tag or whatever) alone, without limiting by type.

Eventi.on('group:', function(e) {
  console.log('Group event: '+e.type, e);
});

This will require a) registering the handler for all known types on the target and b) watching all future listener registrations on the target for new types.

This will have the very obvious limitation that it will be entirely unable to intercept event types not otherwise being listened for by Eventi. Thus, we should never allow/imply support for an entirely empty listener definition (i.e. a "catch-all" listener).