Added an asPlayer method to LocalCommandSender, which casts the object to LocalPlayer if possible and throws an exception otherwise.

src/main/java/com/sk89q/worldedit/PlayerNeededException.java

@@ -0,0 +1,5 @@
+package com.sk89q.worldedit;
+
+public class PlayerNeededException extends WorldEditException {
+    private static final long serialVersionUID = 1L;
+}