Update QP controller

I think we should relate this to the LeggedRobot class in systems/robotInterfaces.  The user should be able to describe the feet only once to be a LeggedRobot, and you should be able to use all that information here (though maybe you want something a little more general, which also declares the hands, etc). 

And even though the QPController is a system, and therefore needs it's own class, you could have a method in LeggedRobot like constructQPController(), which constructs an instance of this class.  

The LeggedRobot class is also going to be the home of all of the ZMP code, etc, so it should be able to set up the QPControllerData for you (directly from the planner, etc)