adds additional methods / classes solely to improve the readability of the code.
addressed Michael's comments on pull request
Added addConstraint method to NLPWCO.
Merge branch 'trajopt' of https://github.com/RussTedrake/drake into RussTedrake-trajopt

Conflicts:
	solvers/trajectoryOptimization/DirectTrajectoryOptimization.m
Restructured constraint class hierarchy