OR-694 Create an Event Message mongoose model
OR-694 Add unit tests
Merge pull request #530 in OR/rse from ~RPIGNOLET/rse:OR-694 to master

* commit '31a3b3c7b9d663f8ea681c05ee94a562178e74a5':
  OR-694 Add unit tests
  OR-694 Create an Event Message mongoose model