Base class with serialization methods for user-defined Python objects.

Classes which inherit from Serializable are enabled with default implementations
of to_json, from_json, __reduce__ (for pickling), and other serialization
helpers. A derived class must either have a member data matching the name of
each argument to __init__ or, alternatively, must provide a user-defined
to_dict() method which returns a dictionary whose keys match the arguments to
__init__.