acarsdec 2.1 released

This is a fix and small improvements release * Fix display of uplink messages * add -A option to display only downlink messages * add -g gain option to set rtl dongle preamp gain * add -p ppm to set ppm frequency correction for rtl dongle * improve -o 0 output (now print the beginning of message ) * use sse2 optimized vector math lib for AM demod (more than 40% CPU load improvement) * other small performance optimizations Download here : https://sourceforge.net/projects/acarsdec/files/latest/download?source=files PS :Acarsdec is a multi-channels acars decoder with built-in rtl_sdr front end.