#551 Email registration should fail if no server email is configured

- Stop registration and issue an error if the email address setting is empty for email registration
- Refactor register command into smaller portions
- Create tests

src/test/java/fr/xephi/authme/output/MessagesIntegrationTest.java

@@ -144,7 +144,7 @@ public class MessagesIntegrationTest {
         messages.send(sender, key, "1234");
 
         // then
-        verify(sender, times(1)).sendMessage(argThat(equalTo("Use /captcha 1234 to solve the captcha")));
+        verify(sender, times(1)).sendMessage("Use /captcha 1234 to solve the captcha");
     }
 
     @Test