add timer/interval functionality to GameObjects
account for GameObject timers in persistence layer

* resume existing timers&intervals when loading an object
* suspend timers&intervals when unloading or deleting an object
* if something goes wrong while writing changes for one object to
  persistence, keep processing the remaining items in the dirty list
  (so issues with one object don't cause dangling timers/intervals on
  others)
add/implement timer/interval related API functions
start/stop onTimePlaying interval on Player login/logout
add API function stub

(...not really related to GameObject timers, just to make tree
harvesting work)
Merge pull request #31 from yelworc/gameobj-timers

GameObject timers&intervals