Add calibration function to support measuring highest current resolution possible (0.1mA) at lowest range (16V, 400mA).

Note that this calibration function is not used by default, since the voltage and current range is so small.  The function is provided as a convenience for anyone that doesn't want to calculate the values themselves.
Merge pull request #6 from tdicola/add_16V_400mA

Add calibration function to support measuring highest current resolution possible (0.1mA) at lowest range (16V, 400mA).