Implemented Orbit Camera Controller

This could use some polish, but it's pretty functional in it's current form. The biggest misgiving I have about the code at the moment is that, unlike the FPS Camera Controller, this one effectively ignores any user-set camera position, and instead overrides it with a value it calculates relative to the focusPosition.

Feature wise I'd really like to tweak the zooming behavior to be more consistent and fluid, as it's probably unsuitable for larger scenes right now.

I've updated the skeletal animation sample to use this controller, as it felt like a better fit (and served as a nice testing platform). I'm happy to revert that bit if you'd prefer.