Add CachedVaults

Only writes vaults to file on server shutdown or player quit, rather
than every inventory close event.

Also caches for read() events.

src/main/java/com/drtshock/playervaults/vaultmanagement/CachedVaultsMap.java

@@ -0,0 +1,16 @@
+package com.drtshock.playervaults.vaultmanagement;
+
+import java.util.HashMap;
+
+import org.bukkit.inventory.Inventory;
+
+@SuppressWarnings("serial")
+public class CachedVaultsMap extends HashMap<Integer, Inventory> {
+    public void setCachedVault(int id, Inventory inventory){
+        this.put(id, inventory);
+    }
+    
+    public Inventory getCachedVault(int id){
+        return this.containsKey(id) ? this.get(id) : null;
+    }
+}