[B+C] Add a method of setting a LivingEntity's goal target. Adds BUKKIT-4256

**The Issue:**
Some living entities, such as wolves, use a different target, the "goal target", to determine what to attack.

**Justification for this PR:**
There is currently no method to set an entity's goal target. Using wolves as an example, a wolf will not attack a player when angry, even when setTarget is used, because EntityWolf checks for the wolf's "goal target".

**Testing results and materials:**
Spawns a wolf and sets its goal target to a player whenever a player joins. Plugin source is included inside the jar.
CraftBukkit - https://dl.dropboxusercontent.com/u/20614340/bukkit/goaltarget/craftbukkit-GoalTarget.jar
Plugin - https://dl.dropboxusercontent.com/u/20614340/bukkit/goaltarget/GoalTargetTest.jar

**Relevant PR:**
B-876 - Bukkit/Bukkit#876

**JIRA Ticket:**
BUKKIT-4256: https://bukkit.atlassian.net/browse/BUKKIT-4256