Add RTC library and examples

[nseidle]

The PR adds a RTC lib that allows users to leverage the 32kHz xtal on most SparkFun boards to track date/time.

• Basic functions: getTime(), setToCompilerTime(), setTime()
• Two examples demo'ing a basic write/read and deep sleep with a CT6 wakeup interrupt for low power measurements

I haven't the chance to measure current consumption of pure RTC.

Future adds: There is an interesting cal routine (12.1.3.1) that could be added as an example to enable end users to calibrate the XT/32kHz circuit to gain even better accuracy.