* Added :autoload to engines path API and redefine usage to be in sync with 6f83a503;
* Do not autoload code in *lib* for applications (now you need to explicitly require them). This makes an application behave closer to an engine (code in lib is still autoloaded for plugins);
* Always autoload code in app/ for engines and plugins. This makes engines behave closer to an application and should allow us to get rid of the unloadable hack required when controllers inside engines inherit from ApplicationController;