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

to be included in drake v 0.9.7
Maybe we should add a third (optional) argument to addStateConstraint specifying which elements of the state to constrain. That way this line could look like:
```matlab
prog = prog.addStateConstraint(BoundingBoxConstraint([-pi/2;el_lo_limit],[pi/2; el_up_limit]), 1:N, [5,7])
```