Added isDestroying/isDestroyed flags meaning and invariants docs.
Enumerable#compact now removes undefined values
Ensure that all view observers are scheduled

Previously Ember.View#registerObserver did not schedule observers as part of
the 'render' queue. In some cases we implemented an additional 'invoker'
that took care of this but it was not universally applied. Here we move the
scheduling into registerObserver itself. This will help prevent unnecessary
DOM updates.
Fix for using {{render}} with same template multiple times, which caused an assertion error