add support for ADC buttons that pulls down to 0, to class BfButtonManager

[mountain-lion-za]

Some shields e.g. the DFR0009 by DFRobot https://wiki.dfrobot.com/LCD_KeyPad_Shield_For_Arduino_SKU__DFR0009 include buttons that pull down to 0 when pressed. This change adds support for this type of button.

The current master code allows the upper bound (resolution) to be set, but the lower bound is hard coded as 100.

Added _adcLowerBound to class BfButtonManager, with method to set it (setADCBounds). _adcLowerBound defaults to 100 (current hard coded value).