As a machine, the REST API should return the elapsed time for the testruns

In the REST API endpoints related with test runs, a new field "duration" should be returned, which would have the calculated difference between the finishedOn and startedOn attributes.

Affected endpoints:

• GET /rest/raven/1.0/api/test/ {key}

  /testruns

• GET /rest/raven/1.0/api/testrun/ and related