Fix getPosition() bug

At present, calling `getPosition()` on a node returns a reference to the node's private `_position` variable. This can lead to mysterious bugs in seemingly simple code. Consider this:

	var pos = node.getPosition();
	pos.x += 5; // bad! this modifies the node's position

This pull request fixes the bug.
Merge pull request #1746 from NatWeiss/patch-7

Fix getPosition() bug