Fix #316 Ordinary player can use /authme unregister

- Change wrong player permissions to admin permissions
- Rename command class names that were the same for the ordinary vs. admin task
- Create test to ensure that commands with OP_ONLY default require an admin permission node

src/test/java/fr/xephi/authme/command/executable/register/RegisterAdminCommandTest.java

@@ -27,7 +27,7 @@ import static org.mockito.Mockito.verify;
 /**
  * Test for {@link RegisterCommand}.
  */
-public class RegisterCommandTest {
+public class RegisterAdminCommandTest {
 
     private static Management managementMock;
     private static Messages messagesMock;