Update FlxObject.hx
Update FlxObject.hx

Added setPositionUsingCenter(), which is more convenient than setPosition() for some situations.

This is my first ever pull request. If this is ok, I have some ideas about introducing 'pivot' points on FlxObjects which generalize the origin and can be used for e.g. setting position, rotating around, connecting with other pivot points.
HaxeFlixel/flixel