Forward Euler and RK4 integrator

This PR defines improves the abstract of the integrator (and discretizer) needed for DDP. Basically, it creates two abstract methods (integrate and discretize) which integration the robot dynamics and compute the discrete time derivatives (fx and fu), respectively. Both methods depend on the desired scheme: forward Euler, RK4, trapezoidal, etc.

The Integrator abstract class aims for high performance computation. Indeed, each scheme has its own internal data which it's created once through createData method.

At the time being, there are two scheme implemented: forward Euler and RK4; they can be used inside our DDP solver. Finally, there are two simple unit-test that checks the discretization of both methods.

@rbudhira and @nmansard have a look of it. Feel free to give me your feedback!