New: the textdomain for translations has changed to the plugin slug in order to meet WordPress standards. Moved settings from Downloads Misc tab to Extensions tab. Added width and height attributes to related images for increased page loading speed. Singleton class.

git-svn-id: https://plugins.svn.wordpress.org/easy-digital-downloads-related-downloads/trunk@904399 b8457f37-d9ea-0310-8a92-e5e31aec5664