Add state and state_reason for Event model
    
    States for Event:
    
    1. Event is created with default state EventState.INITIALIZED
    2. Event state is updated to EventState.BUILDING when some artifacts
    are found and under building.
    3. Event state is updated to EventState.COMPLETE when the event
    is handled successfully.
    4. Event state is updated to EventState.FAILED when there is error
    happens while handling the event.
    5. Event state is updated to EventState.SKIPPED when no action to be
    taken upon the event, e.g. no ArtifactBuild to be built according to the
    blacklist/whitelist policy.
    
    FIXES: #137