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

to be included in drake v 0.9.7
I think we should check if `name` is a string or a cell of strings. If `name` is a string, then the constraints are named `name1`,`name2`,etc. If it is not a string or a cell of strings, then we should throw an error.
```
if(ischar(name))
  name=cellfun(@(a) [name,num2str(a)],num2cell(1:obj.num_cnstr),'UniformOutput',false)';
end
if(~iscellstr(name))
  error('Drake:Constraint:name should be a cell array of string');
end
```