implement APRS display mode for iGates

run with "multimonNG -A" to get APRS format output.

Example output:

	APRS: DO1GL-5>APDR12,DO1GL*,WIDE2-2:=5111.11N/01111.11E$ http://aprsdroid.org/
Merge pull request #9 from ge0rg/master

patch to add correct handling of APRS, used for pymultimonaprs and other possible iGate software