Refactored test bundle reporter.

Changes:
 - Added Reporters group
 - CDRExampleBase records runTime via startDate and endDate
 - Added new methods for CDRExampleReporter as an alterative to KVO:
   - runWillStartExample:
   - runDidFinishExample:
   - runWillStartExampleGroup:
   - runDidFinishExampleGroup:
   - runWillStartSpec:
   - runDidFinishSpec:
   These methods allow more than just observing when state changes: such
   as timing information
 - CDRReportDispatcher wraps cedar's interaction of reporters to
   optionally support new public interface.
 - Test bundle reporter now reports tests as they start and end.
 - Test bundle reporter no longer uses NSDate's description format.
 - CDRExampleBase's run method is now runWithDispatcher: