Whats App presence improvement

this improves the presence indication for Whats App:
- add available to the status when a new "PresenceAvailable" message arrives
- add timestamp to the status when a new "PresenceUnavailable" message arrives
- don't wait 300 seconds until we mark someone as unavailable, whats app tells us that right away
Merge pull request #576 from TitanNano/presence-improvement

Whats App presence improvement