Implement support for DS3231 chip (DS3232 without user writable NVRAM)

Adaption of DS3232RTC code to the DS3231 chip, which lacks the user writable area.

Also, tried to more accurately follow the datasheet for setting the 2 alarms.

There is a lot of code duplication between this and the DS3232RTC library. Should probably see if this can be refactored out.