Add datapath events
Add datapath node_api
Update dispatch_event to call EventHandler#handle_event asynchronously
Unsubsrive the events when the actor is terminated
Datapath event should be handled by the node that it belongs to
Remove a unused method
Link datapath with all managers to terminate them all together
EventHandler should be in vnmgr
Add mock_event_handler to catch the events that webapi dispatch
Implement handlers of datapath events.
Add debug message
Remove debug messages
Merge remote branch 'origin/master' into feature-datapath-event

Conflicts:
	vnet/lib/vnet/node_modules/event_handler.rb
	vnet/lib/vnet/openflow/datapath.rb
Update a spec
Change the response of GET /interfaces
Don't need to unsubscribe the events when vna is shutdowning
Datapath use paranoia plugin
Update to use item.port_number
Item#port_name returns canonical_uuid if port_name is null
DatapathNetwork uses paranoia plugin