Checkign in functioning code for the Arduino and DS3231 and example code

The example runs through and verifies all the provided functions for
the DS3231.   The example also includes suggested connections to your
Arduino.

A good source of the DS3231 is the Chronodot by macetech.com.  The
Chronodot is also availabel from Jameco.
Added blinking of onboard LED so you don't have to hook up external part

This example uses the onbaord D13 pin (on nanos) so you don't have to
add an LED of your own.
Minor edits only

MInor edits only