Added party alliances

Adds the following commands:
`/party alliance`
`/party alliance invite <target>`
`/party alliance accept`

Allies will share party chat and will not be able to harm eachother.
Item and XP sharing is limitied to a single party. Allowing two parties
to team up without sharing resources and XP.

Closes #1009

src/main/java/com/gmail/nossr50/events/party/McMMOPartyAllianceChangeEvent.java

@@ -0,0 +1,90 @@
+package com.gmail.nossr50.events.party;
+
+import org.bukkit.entity.Player;
+import org.bukkit.event.Cancellable;
+import org.bukkit.event.HandlerList;
+import org.bukkit.event.player.PlayerEvent;
+
+public class McMMOPartyAllianceChangeEvent extends PlayerEvent implements Cancellable {
+    private String oldAlly;
+    private String newAlly;
+    private EventReason reason;
+    private boolean cancelled;
+
+    public McMMOPartyAllianceChangeEvent(Player player, String oldAlly, String newAlly, EventReason reason) {
+        super(player);
+
+        if (newAlly != null) {
+            newAlly = newAlly.replace(":", ".");
+        }
+
+        this.oldAlly = oldAlly;
+        this.newAlly = newAlly;
+        this.reason = reason;
+        this.cancelled = false;
+    }
+
+    /**
+     * @return The party being left, or null if the player was not in a party
+     */
+    public String getOldAlly() {
+        return oldAlly;
+    }
+
+    /**
+     * @return The party being joined, or null if the player is not joining a new party
+     */
+    public String getNewAlly() {
+        return newAlly;
+    }
+
+    /**
+     * @return The reason for the event being fired
+     */
+    public EventReason getReason() {
+        return reason;
+    }
+
+    /**
+     * A list of reasons why the event may have been fired
+     */
+    public enum EventReason {
+        /**
+         * Formed an alliance for the first time.
+         */
+        FORMED_ALLIANCE,
+
+        /**
+         * Left a party and did not join a new one.
+         */
+        DISBAND_ALLIANCE,
+
+        /**
+         * Any reason that doesn't fit elsewhere.
+         */
+        CUSTOM;
+    }
+
+    /** Following are required for Cancellable **/
+    @Override
+    public boolean isCancelled() {
+        return cancelled;
+    }
+
+    @Override
+    public void setCancelled(boolean cancelled) {
+        this.cancelled = cancelled;
+    }
+
+    /** Rest of file is required boilerplate for custom events **/
+    private static final HandlerList handlers = new HandlerList();
+
+    @Override
+    public HandlerList getHandlers() {
+        return handlers;
+    }
+
+    public static HandlerList getHandlerList() {
+        return handlers;
+    }
+}