added electrode impedance measurement tool

Ported Lead-Off detection functions from ADSManager.
configure_Zdetect sets fuses in the LOFF register.
changeChannel_Zdetect selects the channel and input for signal
insertion.
minor changes

cleaned up comments and other house-keeping
Update README.md
Update README.md
minor changes

cosmetics
minor changes
Merge branch 'master' of https://github.com/OpenBCI/OpenBCI_8bit
Update README.md
Update OpenBCI_8bit_Device.ino
Merge branch 'master' of https://github.com/OpenBCI/OpenBCI_8bit into OpenBCI-master
Merge branch 'OpenBCI-master'