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.
T_span (which you use everywhere) is probably the wrong name for this.  Everywhere else in the code (and in matlab) tspan means [t0,tf].  I see now that you were being clever by using T_span instead, but until I read the comments, I assumed you mean tspan.