[HAO-Tracker-2014-Primary] Added preliminary support for AD7995 ADC

- added AD7995 class, acting as a driver for AD7995 ADC component. API
is defined (AD7995.h) but the implementation (AD7995.cpp) is for the
moment some kind of a mock, just returning 0
- added AD7995AnalogSensor class, as a subclass of AnalogSensor,
allowing to get values from analog sensors attached to an AD7995