Here's an idea for delta

I noticed that in unity there is a fixed update and a onEnterFrame type update.

Instead of using a set delta, I'm calculating the difference of time between the calls.  Also I made it so you can define fps instead of delta.
Please can we keep `delta` here and let the caller handle the real time values?

If we want to write tests for this, it would be nice to be able to pass in fixed delta values.