Configuration to disable saving of offline players, bugfixing

Edits made to online players will still work. Edits made to offline players will work if they log in before the ISpecialPlayerInventory is unloaded (currently before the plugin unloads).
Closes #6
Refactored IPlayerDataManager to PlayerDataManager, it's no longer an interface.
Fixed a bug with players' online state initially being inverted

src/main/java/com/lishid/openinv/internal/PlayerDataManager.java

@@ -0,0 +1,33 @@
+/*
+ * Copyright (C) 2011-2014 lishid.  All rights reserved.
+ * 
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU General Public License as published by
+ * the Free Software Foundation,  version 3.
+ * 
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU General Public License for more details.
+ * 
+ * You should have received a copy of the GNU General Public License
+ * along with this program. If not, see <http://www.gnu.org/licenses/>.
+ */
+
+package com.lishid.openinv.internal;
+
+import org.bukkit.OfflinePlayer;
+import org.bukkit.entity.Player;
+
+public abstract class PlayerDataManager {
+    public final Player loadPlayer(OfflinePlayer offline) {
+        if (offline.isOnline()) {
+            return offline.getPlayer();
+        }
+        return this.loadOfflinePlayer(offline);
+    }
+
+    protected abstract Player loadOfflinePlayer(OfflinePlayer offline);
+
+    public abstract String getPlayerDataID(OfflinePlayer player);
+}