18650 / LiIon capacity checker

This is based on the work from Adam Welch http://adamwelch.co.uk/2016/01/lithium-ion-18650-battery-capacity-checker/ with several changes

• Ability to discharge 4 cells instead of 1
• Arduino code re-factored to use a millis() timer rather than delay()
• LCD code removed to maximize available Arduino pins
• Logging is done over serial to a Raspberry Pi
• A script runs on the Raspberry Pi to send a tweet when all cells have been discharged

Features I'm looking to add:

• JSON output
• REST API
• Ability to replace individual cells that are complete

Feel free to submit a pull request.

Thanks.