new feature: show password as qrcode (requires qrencode)

useful to transfer passwords to mobile devices.
![qrcode](https://cloud.githubusercontent.com/assets/1571696/4780706/7bb5545a-5c74-11e4-8e1c-abf760cf2b74.png)


Password passed as an argument is visible in /proc, htop, etc.! Please use pipes and the ability of qrcode to read input from stdin instead:

    $GPG -d "${GPG_OPTS[@]}" "$passfile" | head -n 1 | qrencode -t utf-8

Additionally, for empty password it will yield:

    No input data.

which is more informative than exiting without an error message.