Bugfixes

Person objects were not being removed from the floor when an elevator
arrived, leading to the same person object being in two places at once
(or more). Also made the Floor class more thread-safe by making more of
its methods synchronized

This behavior was tested by letting the simulation run and doing a
textual search for each Person. They should appear three times in the
log:

1) When they are created
2) When they enter an elevator
3) When they arrive at their destination and leave the elevator