Added a "step" timer to the Timer class. When starting the timer with a
stepTime value greater than 0, the class will track how many times that
number of ticks (milliseconds) transpire between calls to the new step
function.  If no stepTime value (or a negative value) is given, then the
step() method simply returns 0.

Modified the MainMenu class to use the Timer class via it's new Step
implimentation.

NOTE: The name of the variables that manage step timing... even the name
"step time" may be a bit confusing to look at. Sorry.