Trajectory Optimization

This is not really intended as "ready to accept," but I wanted to start the conversation on trajectory optimization. I added (what I hope is) a pretty flexible framework for direct methods for traj opt, along with some implementations for some first-order integration schemes and direct collocation. The code is far from being speed-optimized, and I think one of the important discussions should be on how to prevent repeated, unnecessary calculations, similar to what Hongkai has with kinsol for inverse kinematics. 

I also added a ConstraintManager class, which is just intended to be a general interface for combining multiple constraints into one. I think this would be a good approach to describe more complex constraints, like support polygons and complementarity constraints.
That makes sense to me. What if I remove all the constraints from the constructor, then add in a number of methods to add constraints (Linear, Nonlinear, Bounded, Managed) as well as some convenience functions for initial and final state constraints that will auto-generate what you need? These sort of convenience functions can be also be added to in the future as we see a need for them.