Add signal-to-noise ratio script.

Signed-off-by: Stefan Westerfeld <stefan@space.twc.de>

src/snr.sh

+# pseudo random pattern
+
+PATTERN=4e1243bd22c66e76c2ba9eddc1f91394e57f9f83
+
+for i in test/T*
+do
+  audiowmark add $i t.wav $PATTERN >/dev/null
+  echo $i $(audiowmark snr $i t.wav)
+done | grep snr | awk '{s += $2; n++} END { print s/n; }'