Fixed exponential scaling plus other minor enhancements

Exponential scaling wasn't calculating correctly for the target 16bit
output value; fixed (and have changed the scale factors for exponential
to appropriate values given this fix). Also removed 'settle' message and
made the drift values more obvious in the debug output. Have updated
various comments also, to make the code easier to follow for anyone
trying to re-engineer to code.
Merge pull request #9 from brumster/master

Fixed exponential scaling plus other minor enhancements