Add event hook support

[mikesir87]

I ended up using tiny-emitter for the event handling. Was very easy to plug in.

To provide a first event, I did a very small refactor to register instances using a new function, which gives a place to publish the instance.new event. To test, you can run something like this...

pwd.newSession([{selector: ".term1"}])
  .on("instance.new", function(instance) {
    console.log("New instance: " + instance.name;
  });

Thoughts?

Resolves #3