[B] Allow inheritance of getHandlerList method. Fixes BUKKIT-4816

The Issue:
getHandlerList must be declared by the event class and cannot be inherited

Justification:
Allowing inheritance of the method can reduce the amount of code in a plugin which has lots of custom events. There may be a reason why getDeclaredMethod is preferred but I couldn't think of one. Relates to my forum post (http://forums.bukkit.org/threads/custom-events.179064/)

JIRA Ticket:
https://bukkit.atlassian.net/browse/BUKKIT-4816