Turn off the more aggressive error correction in receiver.c.

This was generating a lot of highly suspect responses.
Update the gillham and adsblib libraries to correctly handle altitude messages.

The transmitted altitude code has several different formats and may not
use the Gillham code at all. This commit adds support for the altitude
codes as transmitted in mode S altitude responses and ADS-B messages.
Add the beginnings of the aircraft database code.

The eventual aim is to have a database of aircraft objects which contain
all the known information about each aircraft. The objects will be passed
mode S / ADS-B messages and will update their status accordingly.

This commit adds aircraft.py which contains a basic template of a simple
aircraft object and db_adsb.py. The latter is like dump_adsb.py except
that it builds a dictionary of aircraft objects and dumps the accumulated
data from each one at EOF. Both of these files are works in progress.