Version 2.0

* Major changes to plugin architecture;
* Uses a hidden taxonomy to tag posts as Premium;
* Uses a singleton class structure for namespacing;
* Added checkbox to Quick Edit menu;
* Added admin column to posts, to see at a glance which posts are Premium;
* Removed is_premium_post() restriction that required is_single() == true;
* Added $post_id parameter to is_premium_post() so it can be called on any post.

TODO:
* Add POT file for localization;
* Update readme file;
* Add screenshots.