Implement element-left-document handling.

Refactor:
NEW Binding.enteredDocumentCallback(node)
NEW Binding.leftDocumentCallback(node)
NEW Binding#attahedCallback()
NEW Binding#detahedCallback()
Binding#onenter ->  Binding#enteredDocumentCallback
Binding#onleave -> Binding#leftDocumentCallback
Node monitoring is based on document not document.body