Improve Payment module

by Ryan Dao for SilverStripe

The Payment module is an essential part of many SilverStripe e-commerce solutions. It is a powerful stand-alone module, which can be incorporated to SilverStripe websites to enable processing payment for any DataObject. However, despite being widely used, the Payment module is currently facing some limitations such as the lack of unit testing and unstandardized API. This project aims to address those limitations.