Refactored db-part of buy_model to purchase_model

Refactored db-part of buy_model to purchase_model
Also set up controller for handling digital downloads.

    application/config/routes.php
    application/controllers/buy.php
    application/controllers/download.php   (+)
    application/models/buy_model.php
    application/models/purchase_model.php   (+)
    application/views/pages/buy_invalidPost.php
    application/views/pages/download.php   (+)
Created secure access to commerical files

Created secure access to commerical files
Files can now be accessed with the URL
 index.php/download/getfile/[ALBUM]/?id=[ID]&format=[FORMAT]

This checks database to confirm the the purchase.

If available, it attempts to use mod_xsendfile within the CodeIgniter framework.
If not aviable, it uses a fallback method, getfile.php, which uses fpassthru.
This also performs validation, and does not rely on CodeIgniter, as it would
exhaust memory limits.

When doing this completely separate from CodeIgniter, it seems the server is
smart enough to avoid this problem.

File names and paths are also encrypted, and are stored in a configuration file
application/config/downloadconfig (not handled by this repository). See
generate/gen-orig-downloadconfig.php for how this file was generated.

    .htaccess
    README.md
    application/config/orig-secureconfig   (+)
    application/controllers/download.php
    application/models/purchase_model.php
    application/views/pages/download.php   (-)
    application/views/pages/getfile.php    (+)
    application/views/pages/getfilebad.php (+)
    generate/gen-orig-downloadconfig.php   (+)
    getfile.php                            (+)