#437 Email uniqueness in admin command; finalization

- Check also in admin command that email is not already used
- Misc bug fixing (logic errors, changes lost during large merge)
- Use "email" and "setemail" as main labels for /authme subcommands

src/main/java/fr/xephi/authme/command/executable/authme/SetEmailCommand.java

@@ -33,6 +33,9 @@ public class SetEmailCommand implements ExecutableCommand {
                 if (auth == null) {
                     commandService.send(sender, MessageKey.UNKNOWN_USER);
                     return;
+                } else if (commandService.getDataSource().isEmailStored(playerEmail)) {
+                    commandService.send(sender, MessageKey.EMAIL_ALREADY_USED_ERROR);
+                    return;
                 }
 
                 // Set the email address