Corrected incorrect use of GPIO.cleanup()

GPIO.cleanup() only affects ports opened by this program, hence using
it at the top does nothing. For simplicity, it’s better here just to
switch off warnings. Fuller, better explanation here
http://raspi.tv/2013/rpi-gpio-basics-3-how-to-exit-gpio-programs-cleanly
-avoid-warnings-and-protect-your-pi
Merge pull request #1 from raspitv/master

Correct use of GPIO.cleanup()