ERXKey.Type and NSArray#valueForKey(ERXKey) additions

• Added "Operator", "NonModelAttribute", "NonModelToOneRelationship" and "NonModelToManyRelationship" Types to ERXKey and new constructors to allow them to be set when declared. (for example, by modifying the _Entity eogenerator template)

• Added valueForKey(ERXKey) to NSArray. This allows for type-safe valueForKey calls. Defaults to automatically flattens, makes distinct, and removes null values from the resulting array. This improves encapsulation and eliminates common errors caused by needing to write boilerplate code to check if the resulting array is in fact an array of arrays, or if there are null or duplicate elements.