Improved message detection and --stats option.

The demodulation algorithm now only skips the current message if the
CRC is valid, this improves the amount of messages detected with good
CRC by ~ 4% since sometimes we are just a few samples out of sync.

This also improves the amount of messages that can be fixed.

The --stats option was added in order to better evaluate how the
algorithm performs on the same input after some change. So if you run
the program with both --ifile and --stats no Mode S message is logged
at all, but at the end of the processing the program shows you the
amount of messages decoded with good, bad crc, bit corrections, valid
preambles and so forth.