Add RTCM2 support for binary payloads in AIS message type 17

[ianfixes]

This picks up on part of #14 where the binary payload (represented as HEX) seemed to have some field separations:

7c6450cf2004009cfd3d0dff64033daaaa
7c6450e42004009bfd3d0dff65033daaaa
7c64511e20040099fd3d0dff67033daaaa
7c64518020040097fd3d0dff69033daaaa
7c6451b820040095fd3d0dff6b033daaaa
     ^^^       ^         ^

Documentation suggests this is RTCM2-format. This issue tracks the addition of RTCM2 decoding, whether natively or (hopefully) by adopting a ruby gem that is maintained elsewhere.