Fixes #30 Provide Event on open and error event

This pull request provides simple event having only type property for open and error event. I thought to implement Event interface but it requires so many additional things or dependency on something like jsdom to call 'document.createEvent'.

So I fixed it by creating an Event object like MessageEvent. Also, since the error event is only just Event having no property explaining the close reason, I commented them.

Let me know how you think.