Add isInitialized method to PermissionsResolverManager for an easy way to check if the PermissionsResolverManager has been initialized

src/main/java/com/sk89q/wepif/PermissionsResolverManager.java

@@ -64,13 +64,17 @@ public class PermissionsResolverManager implements PermissionsResolver {
     private static PermissionsResolverManager instance;
 
     public static void initialize(Plugin plugin) {
-        if (instance == null) {
+        if (!isInitialized()) {
             instance = new PermissionsResolverManager(plugin);
         }
     }
 
+    public static boolean isInitialized() {
+        return instance != null;
+    }
+
     public static PermissionsResolverManager getInstance() {
-        if (instance == null) {
+        if (!isInitialized()) {
             throw new WEPIFRuntimeException("WEPIF has not yet been initialized!");
         }
         return instance;