Init GPRMC data type

Initialize datatype just in case, prepare for removing prefixed data
requirement
Enhance GPRMC, remove prefixed data requirement

Add additional setter for IP that can remove the socket's leading slash
if necessary. Method is overloaded for backward compatibility. Check if
split on GPRMC sentence has more than one entry, if so, process the
prefix data, otherwise, ignore it (UNTESTED). Updated README.