Enhanced reusability of sensors module

- sensors module is no longer called sesnors_module but
analogSensorModule
- number of sensors is defined (in defs.h) as a constant
- the only visible function "readSensor" allows to read a sensor value
given its number
- customFrameBuilder has been rewritten to be independant of sensors
type and amount
- kiwiFrameBuilder has been modified to be compliant with analog sensors
module changes
- application code has been modified according to this refactoring