E-commerce actions tracking

This PR contains integration of features for tracking checkouts, products added to the cart and products removed from the cart. The involved trackers are GTM, Ad4Push and Adjust. Models have been created in order to map information and share them among different trackers. Related tests have been added as well.
We discussed about it together with Yekmer. Implementing pending queue makes everything a bit more complex. For the moment we will try the keep the library implementation simpler and we will test it during the first integration in next project.