Remove requirement of NMEA talker ID being 'GP'

Since some GNSS-devices sends NMEA data that uses an talker ID other than 'GP' by default, they not accepted by the parser without being configured. This patch allows parsing of streams which identifies differently.

From http://gpsd.berlios.de/NMEA.txt:
"NMEA sentences do [...] include a "talker ID" a two-characterprefix that identifies the type of the transmitting unit.  By far the most common talker ID is "GP", identifying a generic GPS"