Partial revert of 922ed043e9b43ab4118687c7d5d452022f7cbe8a.  Merged the
public and package-private overloads of TimeCard.startJob() and
stopActiveJob() to mirror the functionality of the corresponding methods
in Job.  It was originally intended that package-private versions be
provided to allow tests to control the exact start and stop time of a
job.  However, this requirement is needed by tests outside the model
package, therefore these methods must be public.