[HAO-Tracker-2014-Primary] Turned AnalogSensor class to an abstract
classe

- AnalogSensor is now an abstract class, allowing to handle any analog
sensor (supposed to be read as an int value) 
- AnalogChannelAnalogSensor (formerly AnalogSensor) inherits
AnalogSensog and represents an anlog sensor connected to one Arduino
builtin analog channel