Fixed:  Entities can only be added to the world once per update.
Fixed:  Entities can only be added to the world once per update.

If an entity were to be added like:

var e:Entity = new Entity();
add(e);
add(e);

An endless loop was caused because _updateNext in that Entity would
reference itself.

wselken/HaxePunk,bendmorris/HaxePunk,scriptorum/HaxePunk,scriptorum/HaxePunk,bendmorris/haxepop,HaxePunk/HaxePunk,Pinpickle/Punkiversal,MattTuttle/HaxePunk