HMC5883 async driver

A rust embedded-hal driver for the Honeywell HMC5883. Forked from the original work of Todd Stellanova and made async.

HMC5883 only support a single interface (I2C).

Example

You can connect to the HMC5883 through I2C:

```rust use hmc5883_async::*; let mut hmc = HMC5883::new(i2c);

hmc.init(&mut Delay).await.expect("init failed");

loop {
    if let Ok(temp) = hmc.get_temperature().await {
        info!("Temperature: {:?}", temp);
    }
    match hmc.get_mag_vector().await {
        Ok(mag) => info!("Magnitude vector: {:?}", mag),
        Err(E) => info!("Printing Error {}", E),
    }

    Timer::after(Duration::from_secs(3)).await;
}

```

To run the example:

console $ cd examples $ cargo rb compass