kalibrate-rtl: new package

kalibrate-rtl calculates the local oscillator frequency offset in
RTL-SDR devices.

kalibrate-rtl has no tags/releases, so I'm using the latest commit from
git master (dated 2013-12-14). I made an upstream issue about making a
release back in May[1], but I've gotten no response yet.

[1] https://github.com/steve-m/kalibrate-rtl/issues/7