#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/test/java/fr/xephi/authme/util/ServerUtilsTest.java

@@ -0,0 +1,19 @@
+package fr.xephi.authme.util;
+
+import org.junit.Test;
+
+import static org.hamcrest.Matchers.equalTo;
+import static org.junit.Assert.assertThat;
+
+/**
+ * Test for {@link ServerUtils}.
+ */
+public class ServerUtilsTest {
+
+    @Test
+    public void shouldReturnTrueForSpigotImplementation() {
+        // Spigot is a provided dependency of the project, so the ClassLoader knows about it
+        assertThat(ServerUtils.isSpigot(), equalTo(true));
+    }
+
+}