[C+B] Add the player sample list to ServerListPingEvent. Fixes BUKKIT-5121.

##### The Issue:
Hovering over the the amount of players online in the server list does not display the message which shows players currently logged into the server.

##### Justification for this PR:
This pull request fixes an issue where CraftBukkit lacks something the vanilla server has, and adds a hook for plugins to customize the list returned.

##### PR Breakdown:
The added code creates a new field in ServerListPingEvent, `playerSample`, and one method to access it, `getPlayerSample`. 

This PR is accompanied by a companion CraftBukkit PR. 
 
##### Testing Results and Materials:
Tested with small plugin that removes from the player list people who request it. Everything works as expected.
https://gist.github.com/nightpool/7922846

##### CraftBukkit PR: 
CB-1294 - https://github.com/Bukkit/CraftBukkit/pull/1294
 
##### JIRA Ticket: 
BUKKIT-5121 - https://bukkit.atlassian.net/browse/BUKKIT-5121