Application update

We need to build a system that allows the user to easily update the application. This will also require setting up a canonical repository (I don't want to do that on this server as it is just for development, not for actual releases). GitHub would be an option, although the bookshelf organization name is already taken :(

Some resources:

• http://maxmorgandesign.com/simple_php_auto_update_system/
• https://php.net/manual/en/function.ignore-user-abort.php