do not panic if device is not found

use f64 to calculate frequency_hz in example code

use num::Complex for IQ data

fix cargo git link

add example how to listen FM radio

use sync::mpsc channel for transferring samples from callback to iterator

set version to 0.2.0

add stop_rx

example airspyrx.rs now outputs IQ data to stdout

add iterator for getting samples

make macro for "impl AirspySupportedType for ..", add generic rawbuf_to_vec(..) method to AirspySupportedType trait

make Airspy generic and add start_rx method

add set_frequency

add set_lna_gain, set_mixer_gain and set_vga_gain

fix set_samplerate again and add set_bias_tee method