Add overvoltage protection to thermistor

based on nophead, bobc, NoobMan, Cefiar, and STB's discussion at
http://forums.reprap.org/read.php?13,197920,203564#msg-203564
rework driver passives to make room for barcode

removed resistor on reset/sleep
Change bulk caps to PTH
cleanup routing under 2560
add 10uH to AVCC, better location/val decoupling

for 2560, misc rerouting
add UL and rohs silk instructions to document