Add a way to show the user notifications

[curran]

It should be easy for components to show notifications to the user.

Example notifications include:

• Loading dataset "foo"
• Dataset "foo" failed to load
• Other random messages, warnings, or errors

One possible solution is to adopt a simple convention for Chiasm components to broadcast messages (e.g. setting chiasm.log), then implement a Chiasm component that will render the notifications (pop them up, then fade them out after a few seconds). This component can listen for errors with chiasm.on("log", function (log){ ... });, using the synchronous model.on functionality of model.js.