Refactorings - prepare to remove FoundCommandResult

- Reduce tight coupling by passing list of available commands as param to CommandHandler
- Split command processing method into several smaller ones
- Remove unfinished logic for unlimited command arguments (reason: was not implemented and not used, probably needs another way to handle it once the need for it arises)
- Create test for CommandHandler

src/main/java/fr/xephi/authme/command/CommandUtils.java

@@ -13,9 +13,7 @@ public final class CommandUtils {
     }
 
     public static int getMaxNumberOfArguments(CommandDescription command) {
-        return command.hasMaximumArguments()
-            ? command.getArguments().size()
-            : -1;
+        return command.getArguments().size();
     }
 
 }