Merge ListUtil into StringUtil; refactor HelpSyntaxHelper + create test

The HelpSyntaxHelper had suppressed warnings for string concatenation within StringBuilder - the point of the StringBuilder is that it is faster when you use it to concatenate many elements. If you still use string concatenation with + within these calls it beats the purpose.
(cherry picked from commit bb00be2)

src/test/java/fr/xephi/authme/command/CommandPartsTest.java

@@ -0,0 +1,38 @@
+package fr.xephi.authme.command;
+
+import org.junit.Test;
+
+import java.util.Arrays;
+
+import static org.hamcrest.Matchers.equalTo;
+import static org.junit.Assert.assertThat;
+
+/**
+ * Test for {@link CommandParts}.
+ */
+public class CommandPartsTest {
+
+    @Test
+    public void shouldPrintPartsForStringRepresentation() {
+        // given
+        CommandParts parts = new CommandParts(Arrays.asList("some", "parts", "for", "test"));
+
+        // when
+        String str = parts.toString();
+
+        // then
+        assertThat(str, equalTo("some parts for test"));
+    }
+
+    @Test
+    public void shouldPrintEmptyStringForNoArguments() {
+        // given
+        CommandParts parts = new CommandParts();
+
+        // when
+        String str = parts.toString();
+
+        // then
+        assertThat(str, equalTo(""));
+    }
+}