Include running periods in duration

StopwatchEvent:
  - method getDuration() now includes periods that are not stopped yet

StopwatchEventTest:
  - added testDurationBeforeStop()
Allow retrieving unstopped stopwatch events

Section:
  - added method getEvent()

Stopwatch:
  - added method getEvent()

StopwatchTest:
  - modified testStart() to test for getEvent() method
  - added testUnknownEvent()