Ordered version of NSDictionary backed by a NSOrderedSet.
The main purpose of this class is to provide a guaranteed order to a NSDictionary as the keys are set on the dictionary. The standard NSDictionary does not keep the order of items as they are entered, where as RDHOrderedDictionary uses a NSOrderedSet as its key storage. As well as meaning that the key order is kept constant, it also means various methods are available such as firstKey and firstValue (also lastKey and lastValue) for convenient accessing of entries.