Added missing label error and support for more labelling techniques

Adding a clear missing label error message to make it easy to understand what is going wrong if you have not provided a label.

I've also added support for more valid input labelling options.

This fixes this issue:
#2

core/getters/get_label.js

@@ -2,12 +2,54 @@
 var get_ancestors = require('./get_ancestors');
 
 module.exports = function get_label ($input) {
-	var $forLabel = document.querySelector('label[for="'+$input.id+'"]');
-	if ($forLabel) return $forLabel;
 
+	var labelText =
+		aria_labelledby($input) ||
+		aria_label($input) ||
+		for_attribute($input) ||
+		label_wrapper_element($input) ||
+		title_attribute($input);
+
+	if (labelText) return labelText;
+
+	console.error('Label text for input not found.', $input);
+	throw new Error('Cannot polyfill time input due to a missing label.');
+}
+
+function aria_labelledby($input){
+	var ariaLabelByID = $input.getAttribute('aria-labelledby');
+	if (ariaLabelByID) {
+		var $ariaLabelBy = document.getElementById(ariaLabelByID);
+		if ($ariaLabelBy) return $ariaLabelBy.textContent;
+	}
+	return false;
+}
+
+function aria_label($input){
+	var ariaLabel = $input.getAttribute('aria-label');
+	if (ariaLabel) return ariaLabel;
+	return false;
+}
+
+function for_attribute($input){
+	if ($input.id) {
+		var $forLabel = document.querySelector('label[for="'+$input.id+'"]');
+		if ($forLabel) return $forLabel.textContent;
+	}
+	return false;
+}
+
+function label_wrapper_element($input){
 	var ancestors = get_ancestors($input);
 	var $parentLabel = ancestors.filter(function($elem){
 		return $elem.nodeName === 'LABEL';
 	})[0];
-	return $parentLabel;
+	if ($parentLabel) return $parentLabel.textContent;
+	return false
+}
+
+function title_attribute($input){
+	var titleLabel = $input.getAttribute('title');
+	if (titleLabel) return titleLabel;
+	return false
 }

index.js

@@ -26,7 +26,7 @@ function TimePolyfill($input) {
 		accessibility_block_created = true;
 	}
 
-	var label = get_label($input).textContent;
+	var label = get_label($input);
 
 	$input.polyfill = {
 		$a11y: $a11y,