Add support for Enum classes

[tcalmant]

It seems that Enum classes can't be unmarshalled with jsonrpclib. A special treatment should be applied to reload them.

Sample error:

Error instantiating StatusCode: __call__() missing 1 required positional argument: 'value'

Sample JSON-RPC snippet:

{
  "id": "ae15d720-030c-4bd6-b0fa-aa04e71e56ed",
  "method": "service_14.update_status",
  "jsonrpc": "2.0",
  "params": [
    "f72c4f36-3b70-4ea1-85a6-ee6fe482d0d7",
    "bf206cd3-bcd9-4645-8371-0fe871aeef8a",
    {"__jsonclass__": ["beans.StatusCode", []], "_name_": "ERROR", "_value_": 2}
  ]
}