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.
Merge pull request #2185 from lukemelia/avoid-addClass-on-null-element

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