code: public static void addConfig(Config config) {
  		if (config == null) {
  			throw new IllegalArgumentException("Config can not be null");
  		}
  		if (configNameToConfig.containsKey(config.getName())) {
  			throw new IllegalArgumentException("Config already exists: " + config.getName());
  		}
  		
  		configNameToConfig.put(config.getName(), config);
  		
  		/** 
  		 * Replace the main config if current config name is MAIN_CONFIG_NAME
  		 */
  		if (MAIN_CONFIG_NAME.equals(config.getName())) {
  			DbKit.config = config;
  			Db.init(DbKit.config.getName());
  		}
  		
  		/**
  		 * The configName may not be MAIN_CONFIG_NAME,
  		 * the main config have to set the first comming Config if it is null
  		 */
  		if (DbKit.config == null) {
  			DbKit.config = config;
  			Db.init(DbKit.config.getName());
  		}
  	}
docs: Add Config object
  @param config the Config contains DataSource, Dialect and so on