[B+C]Adds the ability to look up online players by UUID. Adds BUKKIT-5071

The Issue:
With the move to UUIDs as the static player identification method it is necessary for any plugin which uses persistent player data to keep track of players by their UUID instead of their username.  Currently looking up a player by their UUID requires the plugin to get the list of online players then test the UUID it is looking for against every player.

Justification for this PR:
Allows online player lookup by UUID in the same manner as the method plugin developers are familiar with.

Previously: 
-Player getPlayer(String name); was the only way to get a single Player.

Now: 
-Player getPlayer(String name); can still be used by plugins which do not need to persist data or protect against name changes.
-Server.getPlayer(String name); allows plugins which use player UUIDs to get players directly.  This will mostly be use by plugins which persist player data.


PR Breakdown:
Adds the method, Player getPlayer(UUID uuid); to org.bukkit.craftbukkit.CraftServer

Testing Results and Materials:
Test Plugin jar file: https://github.com/Renlar/BukkitTest/tree/master/TestPlugins
Test Plugin source: https://github.com/Renlar/BukkitTest/blob/master/src/main/java/com/liddev/bukkittest/BukkitTest.java

Results test plugin successfully looks up player by UUID and prints player name and player UUID to console.

Relevant PR(s):
B-981 - https://github.com/Bukkit/Bukkit/pull/981 - API addition

JIRA Ticket:
BUKKIT-5071 - https://bukkit.atlassian.net/browse/BUKKIT-5071