Added alarm functionality to DS3234 example

The DS3234 example now contains an example of how to read, set, and
enable an alarm.
Added method which returns date as a string in Big Endian format

Added method toYMDString based on the existing toString. which only
differs in that the format is YYYY/MM/DD HH:mm:ss.

Also added some comments which clear up which registers are being
manipulated based on datasheet.