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.

bitpay / prestashop-plugin