Remove requirement of NMEA talker ID equaling 'GP'

Since some GNSS receivers sends NMEA data that uses a talker ID other than 'GP' by default, they are not accepted by the TauLabs NMEA parser. This patch allows those streams to be parsed using the current functions.

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"