Spin not stable

I tried to use

```
$ node repl.js
// Make the drone takeoff
drone> takeoff()
true
// Wait for the drone to takeoff
drone> clockwise(0.5)
0.5
// Let the drone spin for a while
drone> land()
true
```

The clockwise function cause the drone behave very strange.
It doesn't rotate at the original place. Instead there is a huge drift along side with rotation.