Add a function to get timestamp in seconds since epoch

The DateTime class only stores dates from 2000 onwards, so we simply
add an offset to the number of seconds since 2000-01-01
Remove unused Sodaq_DS3231 class variables

Also make now() not static. There doesn't seem to be any advantage to
keep it static. We always have a class instance to use it. That's how
"wiring" is supposed to be.
Add a function to set the RTC with a epoch timestamp