Add error/completion handlers for sessions, publishers, subscribers

[aiham]

Resolves #7

• Adds onError and onConnect to OTSession
• Adds onError and onConnect to createSession
• Adds onError, onInit and onPublish to OTPublisher
• Adds onError and onSubscribe to OTSubscriber
• Updates example app to display connect, publish and subscribe errors

[aiham]

@Fofir I decided against differentiating between errors from init and publish as implemented in #8 because I don't think you should design your error handling logic based on which stage of the component failed. Actually with your design if you listen to both error handlers and publish is called immediately after initialising the publisher then its possible to get the same error returned via both callbacks. Even a getUserMedia error can be sent through the publish callback. I've made it so only the first callback with an error will bubble up through onError

[Fofir]

@aiham looks cool to me and makes a lot of sense