attach() is now called the first time update() is run so the pin setup
is before attach(). I also removed previous_millis_retrigger to save 4
bytes. Also updated retrigger.ino example with a few more comments.
Updated Constructors to do all the pin() and interval() setups. All the
setup can now be done in one step when instantiating the object. Added
held() and retrigger(). See examples. Updated other examples to use the
new constructor.
Albert Phan