#831 Cleanup, finalization

- Handle AsyncPlayerPreLoginEvent in main PlayerListener class: Spigot emits it in offline mode, whereas offline CraftBukkit doesn't
- Remove listener for sync PlayerPreLoginEvent; not fired by offline CraftBukkit either
- Add warning when offline CraftBukkit is detected and single session setting is enabled

src/main/java/fr/xephi/authme/util/ServerUtils.java

@@ -1,6 +1,9 @@
 package fr.xephi.authme.util;
 
-public class ServerUtils {
+public final class ServerUtils {
+
+    private ServerUtils() {
+    }
 
     /**
      * Check if the server implementation is based on Spigot