Excludes empty directories

I was using the blacklist to exclude the contents of an entire directory, using a Regex like:
```js
var blacklist = /foo\/.*\.js$/;
module.exports = requireDirectory(module, __dirname, blacklist);
```

I was surprised to see that the resulting hash contained:
```js
{foo:[]}
```

Because I was excluding all of the files in the directory, I expected the directory to not appear in the resultant hash. I would similarly not expect and empty directories to not appear. I have made my proposed changes, and added tests to support them.