ITickable, deletion inside advanceTime

Adds a fix for when an ITickable object deletes itself inside the advanceTime procedure.
As discussed [here](http://www.garagegames.com/community/forums/viewthread/134512)

Kudos to David Wyand for [this thread](http://www.garagegames.com/community/forums/viewthread/39389) which explains the issue and helped me alot.