Major modifications in API and implementation

- code has been refactored, leading to a completely new version
- GPS library allows now for valid sentence forwarding and
positioning data extraction, API providing getters for most used NMEA
sentence fields
- GPS has been refactored as an abstract class, containing common code
(NMEA sentence recognition automata, ...)
- GPS2D class has been defined as a subclass of GPS providing 2D
positioning data extraction (time/latitude/longitude/speed/course)
- GPS3D class has been defined as a subclass of GPS2D providing 3D
positioning data extraction  (altitude) and additional data (HDOP,
satellites in use)
- memory usage has been limited (an internal buffer of 85 bytes is used
to store incoming NMEA sentences)
- examples have been modified according to new API