Chai assert.instanceOf fails because it does a toString on MouseEvent.

The reason why it fails in the polyfill is that we copy the native
`MouseEvent.toString` to `WrapperMouseEvent.toString` and that function is not
generic.

https://github.com/toolkitchen/ShadowDOM/issues/59
Some versions of Firefox do not have addEventListener etc on Node.prototype.

Use document.addEventListener instead as the base.
Merge pull request #60 from arv/function-to-string

Chai assert.instanceOf fails because it does a toString on MouseEvent.
Merge pull request #62 from arv/add-event-listener-firefox

Some versions of Firefox do not have addEventListener etc on Node.prototype.
Beef up readme for docs