Added the configuration options
  hourStep (defaults to 1),
  minuteStep (defaults to 1),
  secondStep (defaults to 1).
Each of them determines the number of units the arrows will change the hours,
minutes and seconds, respectively. That is, if hourStep is 4 and the time in
the datetimepicker currently has a hour value of 17, clicking on the up arrow
will change that value to 17+4=21, and clicking on the down arrow will change
it to 17-4=13.
This also wraps around 0 and 24, so that decrementing 00:05:00 with a
minuteStep of 6 will give 23:59:00.

Also adjusted three test cases to reflect the new behavior. Those test cases
expected the click to effect a change of 1, which is the default value, but
they now expect a change of the step values given in the setup function.