Allow fs functions to be called without fs as this arg

The node fs functions can be called without the fs module as the this argument.  This is fairly common practice with things like async:
```js
async.map(paths, fs.stat, function(err, results) {
  ...
});
```

With this change, the mock `stat`, `mkdir`, and `readFile` functions can be called without the mock itself as the `this` arg.