Added buy processing for PayPal (email confirmation)

Added buy processing for PayPal:
  - Processing form data
  - Sending emails to owner/admin/customer
  - Displaying redirection page.

Also did some refactoring.
    application/controllers/pages.php
    application/models/buy_model.php
    application/views/pages/buy_faktura.php
    application/views/pages/buy_paypal.php