Replace Date.now polyfill with performance.now

Also fixed a major frame clamping bug
Adding clearColor option
Prevent browser caching

Adding the urlArg should fix these things
Removed onPreRun API, replaced with onRun.

This is a much cleaner and simpler API. We don't need a thousand
functions to be bound to it, just keep it classy and oldschool with an
`engine.onRun` method.