capture event_id field from OMS external event as sub_type of published OMSDeviceStatusEvent

As agreed in http://etherpad.oceanobservatories.org/pa-r3, 2014-05-22.
rsn oms simulator: include event_id in published event; read fake events from an events.yml; update basic test_external_event_dispatch
oms_simple: use adapted copy of OmsEventListener class from ion/agents/platform/rsn/oms_event_listener.py, which will be updated to listen to OMSDeviceStatusEvent's relayed by the service gateway (instead of setting its own http server)
listen to OMSDeviceStatusEvent's in RSN platform driver to notify agent for handling, in particular for event-driven mission execution purposes
Merge pull request #2131 from carueda/pa_mx_oms_events

M101: OMSDeviceStatusEvent handling triggered from listener in RSN platform driver