Features for non-web game engines

[JosephAustin]

A few features I've added that helped me in a non-document environment with a javascript game engine. Offering them if desirable.

• stepDelta for when you don't need to generate a new unix timestamp, as you already have delta time
• onStep for bodies, so that you can modify it to update your sprites or other graphics automatically. In my case, for example, I need it to set the x and y centers of my sprite and set the rotation in degrees.
• Utility functions to convert between radians and degrees

Thanks for the great work!