Renamed defs.h to pins.h, renamed fields and constants

- changed some constants and fields names to more explicit ones
Moved constants in defs.h 

- these constants are platform-independant/ application-dependant
Refactored setup()

- splitted setup() code into several internal functions, in order to
keep as most as possible a unique abstraction level
- defined GPS baudrate as a constant (in defs.h)