Updated: Allow for multiple methods in each class

This adds an annotation called AbstractEventHandler, which functions just like EventHandler. You still must register the
listener with AbstractEventRegistrar to register abstract events, but you can include multiple per class now. You may
also include normal listeners, but you must register the class with the PluginManager to register those.
AbstractEventRegistrar will only register abstract events.

Technically speaking, AbstractEventRegistrar will register normal events if they're annotated correctly, but please
don't do that. There's no need.