Add possibility to send the product's price including taxes

[luciaebizmarts]

Preconditions

1. Mailchimp for Magento 1.1.15
2. Magento any version.

Steps to reproduce

1. Enable the option 'Include taxe in product price' at the configuration settings of the extension.
   
2. Enable include taxes at the configuration of Magento, to do this follow the link under the box 'Include taxe in product price' at the configuration settings of the extension.
3. Be sure that the product have taxes enabled, please follow this link to do this or create a tax in case that doesn't exist.
4. Click on the button resend ecommerce data if the Mailchimp store already exist.

Expected result

1. See the price of the product on mailchimp with the tax included.

Actual result

1. Only send the price of the product without taxes.

Referenced issue