Support both timer modes

The interrupt may may follow the alarm or timer flag value
or if not in alarm, be pulsed every timer priod.  Enable making
this choice when setting the timer.  Note, the timer flag must
still be cleared manually (it does not pulse).

Signed-off-by: Chris Evich <chris-arduino@anonomail.me>
More cleanup/re-arranging/comment updates

*  Removed all definition head-comments, many were out-of-sync
   with those in header file, and no need to say same thing twice.
*  Re-arranged constructor **after** private BCD converters.
*  Made starting brace use consistent across all definitions.
*  Made consistent two-lines between function definitions.
*  Moved getAlarm() down with older getDate()/getTime() functions.
*  Removed a bunch of disused defines.
*  Spruced up comments to be shorter and more direct.

Signed-off-by: Chris Evich <chris-arduino@anonomail.me>
Make RTC address static and const

It should never ever change.  If it does, it's a problem
the compiler should complain about.

Signed-off-by: Chris Evich <chris-arduino@anonomail.me>