Fixed error in code style
Using serialized form of key ins NSDictionary serialization.
Constants renamed for following code style.
Typo in Serializ(e)able fixed
Compile-time protocol check
Added resource string for NULL representation.
Serialization via [self stringValue]
Changed string representation format
Moved from using self.description
From now serialize:withError: is class method.
Fixed potential null dereference.
Several refactorings.