cleanup trajectory optimization examples (now everything uses the new interface)

to be included in drake v 0.9.7
the old interface took cost(t,x,u), but the new one uses cost(dt,x,u).  in most of the examples, the cost is actually time-invariant, but it's good to switch them over anyhow.

On Oct 12, 2014, at 12:50 PM, Hongkai Dai <notifications@github.com> wrote:

> In examples/DubinsCar/DubinsPlant.m:
> 
> >  
> > -      options.method='dircol';
> > -      %options.grad_test = true;
> > -      
> >        function [g,dg] = cost(t,x,u)
> We have both cost(t,x,u) and cost(dt,x,u). Do we want to stick to one of it?
> 
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