constraints: pretend constraint system.

This is my first attempt at a constraint system (not even a solver) and an example to implement crude touch scrolling. There are some tricky bits even in a simple constraint system like scrolling (the constraints are don't scroll off the top and don't scroll off the bottom). I want the constraint to apply deltas rather than the manipulated value doing it.
constraints: cassowary exploration: incorporate Greg Badros suggestion for fixing relative constraint version.