Allow non-T3 components to listen for messages

[nzakas]

Right now, when a message is broadcast(), only modules can listen for it. That works fine when you're dealing with just T3 components, but sometimes there are non-T3 components that need to be informed of messages as well.

We can implement this quite simply by firing a message event when broadcast() is called. This would allow non-T3 components to do the following:

Box.Application.on('message', function(event) {

    if (event.data.message === 'some_specific_message') {
        // do something
    }

});

So event.data.message is the message name and event.data.messageData is whatever would be passed in as the second argument to onmessage in a module.