Add campaignEcommOrderAdd method v1.3

[kstefan]

http://apidocs.mailchimp.com/api/1.3/campaignecommorderadd.func.php

[basiliscus]

Thank you for the contribution.

I've reviewed the changes. They mostly look good, however I've found the following minor issues:

1. Class EcommOrderInfo

The following fields should be of type Double instead of double:

public double total; public double shipping; public double tax;

And the following field should be of type Date instead of String:

public String order_date;

1. Class EcommOrderItemInfo

The following fields should be of type Double instead of double:

public double qty; public double cost;

And the following fields should be of type Integer instead of int:

public int line_num; public int product_id; public int category_id;

I'll gladly merge the pull request once you correct the issues and confirm the method is tested.

[kstefan]

I fixed field types and retested the method.