Unit tests should also work on slow machines.
Fixes _watchdog start/stop.

Registering _watchdog from the Dispersy constructor allows it to call
database commit before Dispersy.start() is called if the user decides to
wait a minute before 'starting' Dispersy.

_watchdog should be registered and unregistered from Dispersy.start()
and Dispersy.stop(), respectively.
Dynamic callback id's.

Fixes conflicting callback id's when running multiple Dispersy instances
on the same Callback.  This problem could occur since the singletons
were removed and the start/stop was introduced.
Merge pull request #133 from boudewijn-tribler/start-stop

Fixes _watchdog start/stop.