subAll(property, amount, checkAlive, checkVisible)
 Subtracts the amount from the given property on all children in this group. Group.subAll('x', 10) will minus 10 from the child.x value for each child. 
Parameters
Name Type Description property  string  The property to decrement, for example 'body.velocity.x' or 'angle'. amount  number  The amount to subtract from the property. If child.x = 50 then subAll('x', 40) would make child.x = 10. checkAlive  boolean  If true the property will only be