added support for decoding a few AIS packets

see ais_rx.py --help

example: $ ais_rx.py --subdev=A:0 -V 3 -g 65 (debug level 1, failed packets will be also decoded)

OsmoSDR support is under construction

!AIVDM,1,1,,B,13KQH:0OjC1Q4F`T7Ka9u`4P06C0,0*1F
AIS VHF maritime channel 88B (162.025 MHz)
Position Report Class A
Mobile Marine Service Identifier: 230185000
Navigation Status: Under way using engine
Rate of Turn: Right at more than 5° per 30 second
Speed Over Ground: 14.7 knots
Longitude: E 21° 12' 17.400" (21.204833°)
Latitude : N 63° 7' 3.000" (63.117500°)
Distance 9.967 M (18.457 km) and bearing 255.4° to ship
Course Over Ground: 255.0°
True Heading: 258°
Time Stamp: 16 seconds
Receiver Autonomous Integrity Monitoring: RAIM not in use

!AIVDM,1,1,,A,53bEw602=3>u08M0080i4AV0puH4000007P000169@6575Bi08UPC2kk@00300000000,0*73
AIS VHF maritime channel 87B (161.975 MHz)
Static and Voyage Related Data
Mobile Marine Service Identifier: 245727000
AIS version: 0
IMO Number: 9243887
Call Sign: PBGP
Ship Name: LADY NOVA
Dimension to Bow: 28 m
Dimension to Stern: 245 m
Dimension to Port: 32 m
Dimension to Starboard: 16 m
Draught: 3.4 m
Destination: VALKOM
Estimated Time of Arrival 05-05 17:00 UTC

!AIVDM,1,1,,A,402<HcAumAk2<1RTUHT:iCi00H7U,0*57 <- CRC OK!
AIS VHF maritime channel 87B (161.975 MHz)
Base Station Report
Mobile Marine Service Identifier: 2300077
Station Electronic Position Fixing Device: GPS
2013-05-03 19:02:12 UTC
Longitude: E 21° 31' 59.880" (21.533300°)
Latitude : N 63° 12' 31.194" (63.208665°)
Distance 3.089 M (5721 m) and bearing 346.8° to station

Best Regards,
Patrik