fix for grunt-contrib-watch/issues/166 - track new file additions within...

... newly created folders.

Even with these code changes, there's still a use case that the code does not handle. 

If I execute
```
fs.mkdirSync('new_dir');
fs.writeFileSync('new_dir/tmp.js', '');
```
the creation of tmp.js will not trigger any events. This works, however:
```
fs.mkdirSync('new_dir');
setTimeout( function () {
  fs.writeFileSync('new_dir/tmp.js', '');
}, 1000);
```


see: https://github.com/gruntjs/grunt-contrib-watch/issues/166