Fix Wire include + extern definition

The Wire library declares a global 'TwoWire Wire' variable that
clashes when linking to the sketch if Wire.h is included
there as well.  By declaring it as external in this library
header, it ensures proper linking whether or not Wire.h is
also included in the sketch.

Signed-off-by: Chris Evich <chris-arduino@anonomail.me>
Add zeroClock() to zero out date/time

Initializing to anything else requires providing
the initial values to callers for comparison / calculation.
It also potentially complicates weekday/leap year
calculation vs elapsed wall-clock.

Adds a zeroClock() member function that
resets everything to the lowest possible
value/meaning (i.e. 0, 1, 1900). This gives a
simpler point of reference for callers and will
interface better with other widly available date/time
library representations.

The old initClock() is still present with previous values
to maintain backward compatibility.

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