DefaultClock improvement

[remikey]

The DefaultClock is currently merely a while(true) loop.
It does not take the Clock.setTimeUnit() into account and uses a lot of resources when paused.

Please improve this.

Note: make sure that the when the time unit is set to 0, the clock must be in bulk mode (no thread yielding).

[remikey]

Merged into develop:

• setTimeUnit() is now used
• use a ReentrantLock and a Condition to wait when on pause.

[remikey]

What's been done:

• setTimeUnit() is now taken into account.

  Beware: for bulk mode, the user must set the clock to 0.

• use of ReentrantLock and Condition to wait when paused.

[remikey]

Merged into master.