Lots of new features:
- Now logs energy (mWh and mAh)
- Multiple screens (use button to cycle)

Displays: min Voltage, max current, mWh, mAh, peak power, along
with relevant values at each of those points (current at min voltage,
voltage at max current, current and voltage at peak power, etc).

Serial output is unchanged for now, but I'll probably update this
next.
One more screen in big characters

Also now rounds the display of some readings to make screens
more legible.
Cleanups and add a "big reading" Voltage & Current screen
Forgot a "break" statement earlier...
Fix a few typos
Merge pull request #5 from elafargue/multiscreen

Multiscreen support 
Right now mA doesn't show decimals.