[BUGFIX beta] Fix a few bugs in the caching ArrayProxy implementation

[mmun]

• Fix handling of negative indexes: arrayProxy.replace(-1, 0, [obj]);
• Change arrangedContent observer to use private PROPERTY_DID_CHANGE. See the test "should update if content changes while change events are deferred" for an example of why this is needed. This was also causing the ember-data tests to fail.
• Optimize arrayProxy.get('length') so that it no longer triggers the objects cache to revalidate.
• Changed the length computed property to be volatile since we are handling the cache ourselves.
• Added a comment explaining what the this._objectsDirtyIndex field is doing.
  • Switch to using -1 as a sentinel instead of undefined.

I've confirmed locally that this fixes the failing test in ember-data.

[rwjblue]

This will need to be pulled into beta (as the bug it fixes is also in beta)...