Adds tests showing problems working with SC.ChildArray items. Adds test for deep property (more than one level nested) updates.
Tidies nested record unit tests.
Adds tests highlighting the intricacies of how nested records change depending on when and how they're accessed.
Tidies up SC.ChildrenAttribute and removes the call override which doesn't appear to add any value.
Small cleanup of unit test.
Refactors SC.ChildArray to:
 - remove range observers on the underlying data hash array
 - tackle the difficult issue of separating removed nested records from the array and the parent record (fixes failing unit tests)
 - optimizes SC.ChildArray to not be as noisy
 - clean up a bunch of incorrect documentation copied and pasted over from SC.ManyArray
 - hopefully, makes the code more understandable and easy-to-follow
Adds debug-mode only toString method for SC.ChildArray to give a bit more information akin to SC.ManyArray & SC.RecordArray.