Do not always accept HTTP status 200

* The MockMvc instance had been set up to always expect
HTTP status 200 so the tests will fail when testing for requests
that return status codes other than 200
Empty constructors for exception classes

* This allows the exceptions to be thrown without
specifying a message for the stack trace
DTO Refactoring

* Renamed EntityDTO to AbstractDTO. The createEntity
and createModel methods have also been renamed to
createMapFromEntity and createEntityFromMap respectively

* An existing entity can now be updated from a map via the
updateEntityFromMap method. This method performs internal
validation of the source java.util.Map object. The actual copying
of the properties is performed by coptyFromMap; a method that
is not visible to the class calling the DTO instance
Add and delete collaborators API resources

* Implemented the service methods for handling creation
and removal of collaborators

* Additional test data and schema fixes necessary for the
integration tests.