Manual stepping and timer cleanup

Made the method by which Actuate advances its own clocks overridable by
the developer, in case, for instance, you want Actuate to run in sync
with a game clock. I've found Actuate to behave somewhat erratically if
you actually need precise timings.

I also moved the method to get the current system time in seconds to
Actuate.getTimeSeconds(), a dynamic float getter, to allow the developer
to control that behavior as well.

Basic usage is simply to call Actuate.stepTime(delta), which indicates
to Actuate that you're taking charge of time updates.

The hope is that ActuateTimer can be used in place of haxe.Timer
everywhere to leverage the unified clock.