converts DrakeSystem/findFixedPoint over to the new NonlinearOptimizatio...

...n interface.  have not moved RigidBodyManipulator/findFixedPoint over yet, but that will come soon
I think it useful and makes the most sense to have the simple varargin get pushed to all the methods.

i've added documentation which describes it as such.

On Jul 13, 2014, at 11:55 AM, Hongkai Dai <notifications@github.com> wrote:

> In solvers/NonlinearProgramWConstraintObjects.m:
> 
> > @@ -133,7 +133,11 @@
> >      end
> >      
> >      function obj = addConstraint(obj,cnstr,varargin)
> > -      if isa(cnstr,'BoundingBoxConstraint')
> > +      if iscell(cnstr)
> > +        for i=1:numel(cnstr)
> > +          obj = addConstraint(obj,cnstr{i},varargin);
> In this case, what is varargin? If we give a cell of constraints depending on different subsets of decision variables, should varargin be a cell array, with each cell containing a cell array of xind? We should formalize the input argument of addConstraint, since this method is going to be extensively used. Also add documentation for this method
> 
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