Further fix on proper parsing of proprietary messages

[randallpittman]

My fix for issue #44 had the wrong indexes since I foolishly copied it directly from the ubx.py code.

the proper __init__ method should have been the following:

    def __init__(self, manufacturer, data):
        self.sentence_type = manufacturer + data[0]
        super(GRM, self).__init__(manufacturer, data[1:])

This same method (with SRF) should be added to srf.py