/**
 * function to limit textarea fields to a certain lenght
 */
function update_count(field, counter_id, max_length){
	// get counter field
	counter = document.getElementById(counter_id);
	// check length
	if (field.value.length > max_length){
		// shorten length
		field.value = field.value.substring(0,255);
	}
	// update counter value
	counter.value = max_length - field.value.length;
}