Add logging

To make this even more general you could inject the append method for this handler here:
```
// node fs:
var appendFunc = fs.appendFile.bind(fs, path);
// our io":
var appendFunc = io.appendFile.bind(fs, true, path); // async call, won't block

// then:
new FileHandler(appendFunc);
```

maybe it's not even a "FileHandler" in the end anymore.