[Xastir] New script: ads-b.pl for snagging airplane positions

[Jason Godfrey]

FWIW, I got this to work on my Mac. I was able to get dump1090 and the
script working surprisingly easily.

My only problem so far (and it is mostly minor) is that xastir sometimes
doesn't clear the object/label when it is drawing the latest update, so a
bunch of icons and text can pile up on each other. Eventually it seems to
do an update of the map and clear the old icons.

- Jason