Continous saving of the game

This pull request makes the game periodically save the game. At a certain percentage of unloaded chunks, a save gets also triggered to free the memory. 

The saving has two phases: Phase 1 is very short and happens on the main thread. In phase 2 data is written and the game continues to run. During phase 2 a "Saving..." hint is displayed in the debug view at the top right.

See http://forum.terasology.org/threads/continous-saving-of-the-game.1105/ for more details
Maybe this could be refactored into separate methods or even classes?