Correct FET misunderstanding, adds cal port and optional AREF cap

FET terminiology: drain and source do not refer to conventional
current. I assumed they did... and wired it up incorrectly. I also
 added a header to make calibrating max charge current easier.
Finally, I added a footprint for an optional 100nF capacitor to
decouple AREF. Some Arduinos have this onboard... my cheapo clones
never do.