RemoteService: dispatcher UID indicated in service properties

- A constant property is added by the discoverer of an end point
  to the imported service properties: the dispatcher UID
  ('sender' in the end point event packet)
-> this allows to discrimate services according to their exporter
New remote service protocol: JSON-RPC

- Based on jsonrpclib (https://github.com/joshmarshall/jsonrpclib), available on PyPI
- Not fully tested, but seems to work well on Python 2
EventAdmin.Event now supports a default value in get_property

- Event.get_property(key, default): if default is not given, the method raises
  a KeyError exception.
- Added Event.get_topic(), in addition to the topic property, in order to support both
  methods of access