Better support of the spec

* web worker
* Window.postMessage and Worker.postMessage
* Allows message ports to be pass through a user agent
* `MessagePort.start` dispatches events asynchronously
* Events queueing
* When targetOrigin is set to "*", the origin on the event object is "null"
* Message.encodeEvent
* Remove setOnMessage ( prevent the lib to be a polyfill )

[Fixes #1]
Sample to pass a port from a grand parent window to a grand child iframe
Create sample project for web worker
Merge pull request #3 from Cyril-sf/worker

Web worker support and API tuning