Change ExecutableCommand interface (wip – doesn't compile)

- Change interface to use (CommandSender, List<String> arguments)
- Use CommandArgumentDescription#name instead of "label" (to prevent confusion between command labels and arguments)
- Simplify command difference computation in CommandHandler (no longer consider argument difference)

src/test/java/fr/xephi/authme/command/help/HelpSyntaxHelperTest.java

@@ -1,7 +1,6 @@
 package fr.xephi.authme.command.help;
 
 import fr.xephi.authme.command.CommandDescription;
-import fr.xephi.authme.command.CommandParts;
 import fr.xephi.authme.command.ExecutableCommand;
 import fr.xephi.authme.command.executable.authme.RegisterAdminCommand;
 import org.junit.Test;