Added database registering of purchases

Added database registering of purchases, also did some refactoring.
There is now a dedicated controller for the buy process.

    application/config/routes.php
    application/controllers/buy.php             (+)
    application/controllers/pages.php
    application/models/buy_model.php
    application/models/news_model.php
    application/views/pages/buy.php
    application/views/pages/buy_faktura.php
    application/views/pages/buy_invalidPost.php (+)
    application/views/pages/buy_paypal.php
    javascript/buy.js