support bias_t option

[jbrzusto]

some newer rtlsdr dongles include an antenna bias voltage. e.g. here

This one is controlled by setting / clearing gpio 0 in librtlsdr.c:

int rtlsdr_set_bias_tee(rtlsdr_dev_t *dev, int on)
{
    if (!dev)
        return -1;

    rtlsdr_set_gpio_output(dev, 0);
    rtlsdr_set_gpio_bit(dev, 0, on);

    return 1;
}

and then adding a call to force it off from rtlsdr_close:

    // Turn off the bias tee
    //rtlsdr_set_gpio_output(dev, 0);
            rtlsdr_set_gpio_bit(dev, 0, 0);

Implementation for SG:

• [ ] add above call to librtlsdr
• [ ] add a command to set / clear in rtl_tcp
• [ ] add a param in jbrzusto/sensorgnome/master/rtlsdr.js:RTLSDR.prototype.rtltcpCmds
• [ ] add a default ('off') setting in defaultDeployment.txt

[jbrzusto]

a patch is in testing, but I have confirmed it works with the rtl-sdr.com dongles sold by compudata.ca