Woocommerce Aweber Newsletter Subscription 4.0.1

* Fix - Prevent fatal errors when working with an AWeber API object after a failed endpoint request.

* Feature - Subscribe the customers to different (and multiple) AWeber lists depending on the products they purchase during checkout.
* Feature - Add or remove AWeber subscriber tags depending on the purchased products.
* Feature - New widget to display AWeber Web Forms.
* Feature - New and more intuitive settings page.
* Tweak - Remove the AWeber credentials when uninstalling the extension.
* Tweak - Better error handling for the AWeber API requests.
* Dev - Log possible errors in the AWeber API requests.
* Dev - Rewritten the entire extension.

* Tweak - Check if the 'AWeberAPI' class already exists before require it.
* Tweak - Only create a single instance of the 'AWeberAPI' class.
* Fix - Fixed undefined 'subscribe_checkout' index warning in the checkout form.
* Fix - Fixed warnings when there is no any 'Aweber Web Form' available to select in the Aweber widget settings form.