Changed HardwareSerial to FastSerial solved few problems with
SoftwareSerial. Also implemented a sanity check at NazaGPS Receiver.
With that, we dont have any freeze problems anymore and we can use the
SoftwareSerial Port. Meaning we can use also atmega328 for the job :D
Added Serial Port argument to NazaGPS Class to make easy to switch
between MEGA Mode or normal mode.