Single clock library

[awickert]

Merging the BottleLogger and LogMega branches is very much reliant on not having to instantiate two clock libraries; this is a placeholder for this goal.

[awickert]

See what changes @adafruit has made (https://github.com/adafruit/RTClib/blob/master/RTClib.cpp) but keep each RTC chip (or at least RTC chip interface) as its own class that inherits from a core set of functions. (And see if there's any action here: adafruit/RTClib#51.)

[awickert]

A good response to my comment by @MrAlvin that I should include here:
https://github.com/MrAlvin/RTClib

This looks like a good way to streamline the RTC library. But Chad has now done more work with the DS3231 library, so this will be a later enhancement