[B+C] Added event for item frame dropping item. Fixes BUKKIT-5020

##### The Issue:
Minecraft 1.7 drops the item in the item frame first, without this event, world protection plugins have no way to protect against players stealing from item frames.

##### Justification for this PR:
With this PR, plugins now have an event that is fired when an item frame tries to drop an item that can be blocked by canceling it.

##### PR Breakdown:
This PR adds the code for the event to bukkit, in addition an implementation in CraftBukkit is provided. 

##### Testing Results and Materials:
I wrote a simple plugin that will simply cancel all events for item frames dropping items.
A jar containing both the compiled and source files is provided here: https://googledrive.com/host/0BzuEkOAtffh-VVVpbHk4bkZYRFE/HangingEventTest.jar

##### Relevant PR(s): 
CB-#### - https://github.com/Bukkit/CraftBukkit/pull/#### - CraftBukkit component
 
##### JIRA Ticket: 
BUKKIT-5020 - https://bukkit.atlassian.net/browse/BUKKIT-5020