function getRuleIndex(target, field){
		var name = getPluginName(target);
		var rules = $(target)[name]('options').filterRules;
		for(var i=0; i<rules.length; i++){
			if (rules[i].field == field){
				return i;
			}
		}
		return -1;
	}
get filter rule index, return -1 if not found.