Prototype System class.

@moorepants, can add your tests to this branch? From this point I'll write code in a test-driven way. It'd be fun to continue to work on this together, but I'm also happy to do the coding and you can review afterwards (if you're busy, etc.).

- [ ] Edit Scene so that it takes in a System.
- [ ] Make KanesMethod and LagrangesMethod derive from a single abstract base class (ABC), perhaps MechanicsMethod. This ABC will be required to provide certain attributes.
- [ ] Document in Sphinx, README, etc.
This is what I had been implying for re-integrating Scene visualization stuff.
So that we supply `times` and `sys` to the Scene class, and is easier to handle than the existing structure.
I am ready to work on integrating this code with `Scene` class(after tomorrow).

But also there will be some backwards incompatible changes(in Scene class as well), when this is integrated into Scene.