Partially fixes rschmukler/agenda#100

Three tests are failing with this change.

Two are integration tests.  I believe this is because the
.every('one day') call schedules it for the next day rather than
running it once today and then scheduling it.  I'm pretty sure I
can get this working by toying how the job.proto.scheduleNextRunAt
sets the date when no lastRunAt is set.

The other test that is failing (that I'm personally worried about)
is the lock job tests.  The test wants the job.lockedAt to be set
to null, and it is currently being set to undefined.