Integrate clock into existing contracts

[bekauz]

All Tick {} message handlers should validate that the sender of incoming messages is the clock contract.
Also should enqueue (and implicitly dequeue) themselves as needed.

This should also cover the update of tests so that only the clock is getting pinged.

[Art3miX]

#22 Covers this.

1 thing left to do is verify the clock is the caller of the ticks in all contracts.