Implement new integrators

[HugoGranstrom]

At the moment I have implemented a few integrators but I would like to have as many as possible. If you know of any that isn't here please send a pull request. If you want any help I'm glad to be at your service, I know my code isn't the prettiest thing in the world. Would be really interesting if anyone could add a few Runge-Kutta integrators, or just fix mine.