For protobuf messages that contain enum fields, it is now possible to
specify that enum variants should be serialized as names and not as integers.

E.g.

json_str = MyMessage.to_json(my_message, enum_strings=True)

Similarly, serialization from json that uses this convention is now supported.

This is useful for interoperation with other data sources that do use
strings to define enum variants in json serialization; and for
debugging, where visually inspecting data structures can be helpful,
and variant names are more informative than numerical values.

Re-addition of #107 with correct support for nested enum definitions
this time.