Fix issue where classNameBindings can try to update removed DOM element.

Ember.View#registerObserver now uses states to invoke observers only when there is an element for them to operate on. This avoids a class of issues that can arise when observers are triggered even though the element has been removed from the DOM.

Conflicts:
	packages/ember-views/lib/views/view.js
	packages/ember-views/tests/views/view/attribute_bindings_test.js