Updated to work with Prestashop 1.6 and better backward compat. support

* Prestashop 1.6 drastically redesigned some parts of the software from version 1.5.  Specifically, the placement of modules in the admin area and enforcing MVC by completely removing older checkout files.
* Added conditional code to use new MVC arch when required.
* Added official backward compatibility support from the Prestashop developers.
* Fixed some rendering bugs due to the removal of certain javascript & CSS files in 1.6.
* Fixed 404 bug when returning to the cart after paying an invoice.
* Updated the README to reflect new testing and slight changes for installing plugin on 1.6.