Fix $plugin_basename to allow plugin_action_links

With the move to /admin and /public directory, the class-plugin-name-admin.php needed to be changed to use __DIR__ instead of __FILE__ to create the $plugin_basename, otherwise the hook would be something like plugin_action_links_plugin-name/admin/plugin-name.php which doesn't work.
Merge pull request #116 from neverything/patch-1

Fix $plugin_basename to allow plugin_action_links