Update shinken/modules/zmq_broker.py

Added basic support for ZeroMQ topics.
It will use b.type as the topic so the client can filter on message type.
Update shinken/modules/zmq_broker.py

Fixed module imports to import optional modules only when necessary.
Update shinken/modules/zmq_broker.py

Fix whitespace.
Merge pull request #739 from thomascellerier/master

Add topics to ZeroMQ broker