Replace querstring library with visionmedia's qs

Node `querystring` library doesn't handle nested object property
correctly (`stringify` method). For instance:
```
{ id: '1',
  name: 'skill1',
  language: 'en',
  operators: [ { login: 'a@b.pl' }, { login: 'jenny.doe@mycompany.com'
} ],
  status: 'offline' }
```

got parsed into:
```
id=1&name=skill1&language=en&operators=&operators=&status=offline
```

@visionmedia to the rescue!
https://github.com/visionmedia/node-querystring/ handles that correctly.