Enhancements of FlxPath

Inspired by [this forum thread](http://forums.flixel.org/index.php/topic,7240.msg40268.html#msg40268). Seems like `FlxPath` is pretty basic.

- added `pathAutoCenter` flag to `FlxObject`. If false, the object will not
follow the path at its center, but instead at its origin
- added `setPathNode()` function to `FlxObject` to be able to change the
node a path is currently at