Add a HistoryActor and a TaskFailureEventRepository.
Expose task failure in API.
Add a test to check AppFailures deleted correctly.
Cleanup: initial task failure API response

- Fixed logic to NOT count TASK_KILLED as failure
- Renamed TaskFailureEvent => TaskFailure
- Augmented TaskFailure (state, host, taskId, etc)
- Added custom TaskFailure extractor object
- Added TaskFailure protobuf (un)marshaling
- Added JSON (de)serializer for o.a.m.P.TaskID
- Added TaskFailure proto/JSON tests
- Deleted some unused code in HistoryActor
- Added explicit return types to public methods
- Added usage to the REST docs
- Fixes #482
Added lastTaskFailures to GET /v2/apps/{appId}

- Updated REST docs with new response
Merge pull request #686 from mesosphere/wip-task-failures

Latest Task Failure in App Response JSON