Experimental

Improvements to conversion bit-banging:
 * Conversion is now atomic (cannot be interrupted), interrupting the conversion could cause data corruption if data is modified in another interrupt.
 * Shifting, reading and storing each bit is split up to achieve ~50% duty cycle clock signal.

Clock signal produced by an Arduino Duemilanove has been analyzed and shows correct behavior. Needs testing with device before acceptance since I do not have a MAX31855.