Integrate billing module #179
Description
Integrate billing module created by Tin.
The source code currently is https://drive.google.com/file/d/0B5frq7rZIF4leno2ejlYa19OcnM/view?usp=drive_web. This is the backend that runs continuously to check due invoices. It is in Python since PHP code can only run when someone triggered it by requesting a page (PHP is request-based). @prawn9189, you can also still use PHP code by triggering it using cron.
The front end part of Pongo has been modified by @thanyawzinmin, as far as I know. He maybe left it commented so you need to enable it.