Switching to "qs" module instead of node's "querystring" module

### The Problem
Node's built-in querystring module doesn't stringify arrays in the proper way required by oDesk API.

### The solution
Use [node-querystring](https://github.com/visionmedia/node-querystring) library instead.

Here is a small test that show the difference in output.

```js
var qs = require('qs'),
    querystring = require('querystring');;

var query = {
    c1: 'Web Development',
    c2: ['Web Design', 'Web Programming']
};

console.log('node: ' + querystring.stringify(query));
console.log('node-querystring: ' + qs.stringify(query));
```

Output:
```
node: c1=Web%20Development&c2=Web%20Design&c2=Web%20Programming
node-querystring: c1=Web%20Development&c2[0]=Web%20Design&c2[1]=Web%20Programming
```