RTC (Real-time-clock) for emonPi offline usage

RTC (Real Time Clock) module with coin cell battery backup for the emonPi to allow it to work offline without an interent connection. 

By default the emonPi uses the interent to obtain NTP time, if the emonPi is to be used offline without access to an interent connection it's important the time is set correcty. This RTC module will allow the time to be preserved. After installation of RTC module the emonPi shuld be booted once with interent acces to allow it to obtain NTP time, or the time can be set manually via SSH commandline using `sudo raspi-config`. 

Requires soldering to install - if ordering together with an emonPi please request installation before shipping

Note: The system time on the emonPi should always be in UTC, the user timezone should be set in Emoncms on the user account page. 

Installation: https://wiki.openenergymonitor.org/index.php/EmonPi#Adding_a_Real_Time_Clock_.28RTC.29