Test GPIO door functionality

I made a branch in my fork with some test code, but it looks like you should be able to do this:

```
use gpio::{GpioOut};
use std::{thread, time};

/*
On the board,
Pin 87 -> Motor Rev
Pin 89 -> Motor Fwd
*/

/*
Here's how you'd do it in BASH

ON:
# echo 89 > /sys/class/gpio/export
# echo out > /sys/class/gpio/gpio49/direction
# echo 1 > /sys/class/gpio/gpio49/value
*/
fn door_open() {
// Motor Forward for 0.5 seconds.
gpio89.set_value(value).expect("could not set gpio89");
thread::sleep(time::Duration::from_millis(500));
value = !value;
gpio89.set_value(value).expect("could not set gpio89");
}

fn door_close() {
// Motor backwards for 0.5 seconds.
gpio87.set_value(value).expect("could not set gpio87");
thread::sleep(time::Duration::from_millis(500));
value = !value;
gpio87.set_value(value).expect("could not set gpio87");
}

fn yeet_door() {
// Let's try to open GPIO 87 and 89.
let mut gpio87 = gpio::sysfs::SysFsGpioInput::open(87).unwrap();
let mut gpio89 = gpio::sysfs::SysFsGpioOutput::open(89).unwrap();

// GPIO87 and 89 will be toggled every 0.5 sec in the background by a different thread
// This should cause the door to lock and unlock.
let mut value = false;
thread::spawn(move || loop {
// Motor Forward for 0.5 seconds.
gpio89.set_value(value).expect("could not set gpio89");
thread::sleep(time::Duration::from_millis(500));
value = !value;
gpio89.set_value(value).expect("could not set gpio89");
thread::sleep(time::Duration::from_millis(3000)); // Leave it in that state for 3 secs
// Motor backwards for 0.5 seconds.
gpio87.set_value(value).expect("could not set gpio87");
thread::sleep(time::Duration::from_millis(500));
value = !value;
gpio87.set_value(value).expect("could not set gpio87");
thread::sleep(time::Duration::from_millis(3000)); // Leave it in that state for 3 secs
});
}
```

This needs to be tested, though. I haven't gotten a chance. The Beagle appears to be intelligent about its GPIO pin assignment in the OS, so pin 87 should actually be 87, and standard pin assignment libraries should work.