Enable nested query strings for mailchimp export api

The change is very small - I just updated the Mailchimp exportAPI to use the qs node-module for querystring rather than the core node querystring module.  The api expects an object array for segment in the form of:
{match: "all", conditions: [{field: "interests-x", op: "all", value: "SomeValue"}]}

That was not possible to generate using querystring.  

Thanks for the awesome module and let me know if you have any questions!