Fix #716 and alpha color for some FlxSpriteUtil

FlxSpriteGroup support updateMotion

Remove FlxVelocity copy from group to children. It makes no sense. It means that if you had a velocity to the group. Then the group will move and every children will move aswell so children will move twice as much as the group. So it necessary to keep the whole group synchronized. You can still change children velocity by hand.

Also include support alpha channel for some FlxSpriteUtil drawing methods