Fixed ordering bug.

Added two more objects to the demo, so you can see that reordering
by moving something up from below works, but moving the top element
down two winds up with the first two items having the same 'idx'.
This is because Ember sorts the list on every object modification
by default, so elements are moved before they're reindexed.  Adding
the calls to "beginPropertyChanges" and "endPropertyChanges" makes
it work correctly in all cases.