Initial prototype for #212

RACAble with KVO change dictionary
Revise value class interfaces

* Proxy to change dictionary (- KVODictionary)
* Always send aggregate class (RACPropertyChanges)
* Aggregate class to sequence (- rac_sequence)
* Keep aggregate properties (objects, oldObjects, indexSet) for KVO parity
* Keep individual (RACPropertyChange) base properties (kind, prior) for aggregate parity
* Pending implementation