[B+C] Fire a new LockedChestDecayEvent when locked chests decay.  Adds BUKKIT-122

##### The Issue:
There is currently no way to prevent locked chests from decaying.  This pull request is intended to allow plugin developers to do that.

##### Justification for this PR:
Although locked chests "are all but removed from the game at this point", they do not have the chest texture anymore, and BUKKIT-122 was closed as WONTFIX for these reasons, some people may want to still use locked chests:

  * as decoration blocks (this is *my* desired use case); or
  * to do various fun things (e.g. the [LockedChestSurprise](http://dev.bukkit.org/bukkit-plugins/locked-chest-surprise/) plugin created by the reporter of BUKKIT-122).

I implemented a new `LockedChestDecayEvent` as locked chests are obviously not leaves, and I don't think existing plugins that listen for `LeavesDecayEvent` should have to check to see if a block is a locked chest.  If you all would prefer that I fire `LeavesDecayEvent` instead, I would be happy to update my code to do that instead.

##### PR Breakdown:
This pull request:

  * Adds `net.minecraft.server.BlockLockedChest.java` as it wasn't there before.
  * Fires the new `LockedChestDecayEvent` when a locked chest decays.  The code that does this is a direct copy/paste from `BlockLeaves.java`.

The Bukkit pull request creates a new `LockedChestDecayEvent` that will be called by `net.minecraft.server.BlockLockedChest.java:a` when a locked chest decays.  This new event is a direct copy/paste of `LeavesDecayEvent.java`.

##### Testing Results and Materials:
I created a plugin that cancels `LockedChestDecayEvent`s.  I then set up a new CraftBukkit server with [this `server.properties` file](http://pastie.org/8202165) (`online-mode` is set to false because the auth server was down for me.  I've also tested with it set to true.), installed the plugin, and gave myself a locked chest.  I then placed it and waited for it to decay.  The plugin caught the decay event, cancelled it, and the locked chest *did not* decay.  I also tested with the plugin *not* installed, and the locked chest *did* decay.  No other plugins were installed, and no other configuration files were changed.

This plugin also has debugging output.

Plugin source:  http://code.s.zeid.me/lockedchests/src/ec3a6936ae2657c33bd88e9e234adca01c7f0e2d
Plugin binary:  http://code.s.zeid.me/lockedchests/downloads/LockedChests-0.1.jar

##### Relevant PR(s): 
B-913 - https://github.com/Bukkit/Bukkit/pull/913 - Implements the new `LockedChestDecayEvent`.
 
##### JIRA Ticket: 
BUKKIT-122 - https://bukkit.atlassian.net/browse/BUKKIT-122