Added performance.now() and canceled rAF after stop()

[ghost]

Currently, operations after stop().

With the proposed changes.

I took the liberty of adjusting the init/start/stop lifecycle and time handling in function of the rAF callback lifecycle.

• Now the first context.dt is always 0 because there is no previous frame
• context.now and context.dt hold meaningful values only when context.running is true
• context.millis is the overall time spent in the animation loop [1] , not the context's lifetime
• context.st (start time, in the spirit of .dt) was added in order to easily determine the lifetime of the context
• Context initialization was adjusted to the new main behavior

I tried to match your coding style, but some air may be missing here and there. :)

[1] Minus the last frame, which is a shortcoming that might need to be addressed.