Remove Singleton base class

Currently there is no adequate solution to late static binding for <= PHP 5.2 which we need to support because WP does. All the 'solutions' involve using debug_backtrace() or passing the class name to get_instance, i.e. Some_Class::get_instance( 'Some_Class' ); which won't work with static class via WP hooks. Which really leaves us to duplicate get_instance code in each class if we want to use singletons.
Merge branch 'hotfix/2.9.2'