Implement a model and API for events

[ddabble]

A "start event" would create/open a new incident, an "end event" would close the incident, and other events would simply provide a log of things connected to the incident. The latter would possibly include acknowledgements (see #72), which could automatically create an event that they're connected to.

The Event model should probably include these fields:

• A relation to an Incident
• A relation to the SourceSystem the event came from
• A timestamp
• An enum for the type of event (incident_start/incident_end/close/acknowledge/other)
• A description of the event