Specified id and medium as paypal option fields

Specified ID and Medium as paypal option fields.
The medium is still mentioned in the item title.
    application/views/pages/buy_paypal.php
Fixed email configurations

Fixed email configurations
    application/controllers/buy.php
Implemented IPN validation and database registry

Implemented IPN validation and database registry, also sends user email with
download links upon paypal confirmation.

Owner also receives email with information of paypal IPN confirmation.

    application/controllers/download.php
    application/models/buy_model.php
    application/models/purchase_model.php
    application/views/pages/process-ipn.php   (-)