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.
`data_function` here is a property of `NLP`, but I did try this (maybe it's what you mean), and it is "fast" (~8 seconds). However, this assumes that all function handles require the NLP as the first argument.

```
NLP.addDataFunction(@dynamics);
...
NLP.data_function(obj,args{:})
```