Add a function in testNLPWConstraint, so that the solver will use fmincon if snopt is not installed
NonlinearProgram already has the logic to set default solver as fmincon when SNOPT is not installed. Also set 'snopt' and 'fmincon' method as protected in NonlinearProgram to expose less details to the user.
Merge pull request #333 from hongkai-dai/fixTestNLPWConstraint

handle the testNLPWConstraint if snopt is not installed