Baudrate for serialport on osx

[mineichen]

I was able to use your driver on OSX with the following modification of your sample:

let serial_port = serialport::open_with_settings(devicepath, 
        &SerialPortSettings { baud_rate: 115200, ..SerialPortSettings::default()}
    ).unwrap();

Would you like to have this in the examples or might it be a good idea to add a featureflag for an optional serialport dependency and provide a method to create a RplidarDevice from a string directly? I'll create a pull request with your preferred option if you like