fix for sorting elements within a non-homogenus list of elements

For instance, say you are sorting tbody's within a table element.  There
can also exist tfoot and thead within the table element.  The _index()
function assumes that the list of elements within the parent is
homogenus (ie: just li's below an ol/ul).  This isnt the case when
sorting tbody under table.  This is edge case until you want to group
tr's under a table and sort on those groupings.

Sortable.js

@@ -271,7 +271,7 @@
 			}
 
 			// get the index of the dragged element within its parent
-			oldIndex = _index(target);
+			oldIndex = _index(target, options.draggable);
 
 			// Check filter
 			if (typeof filter === 'function') {
@@ -766,7 +766,7 @@
 					_toggleClass(dragEl, this.options.chosenClass, false);
 
 					if (rootEl !== parentEl) {
-						newIndex = _index(dragEl);
+						newIndex = _index(dragEl, options.draggable);
 
 						if (newIndex >= 0) {
 							// drag from one list and drop into another
@@ -786,7 +786,7 @@
 
 						if (dragEl.nextSibling !== nextEl) {
 							// Get the index of the dragged element within its parent
-							newIndex = _index(dragEl);
+							newIndex = _index(dragEl, options.draggable);
 
 							if (newIndex >= 0) {
 								// drag & drop within the same list
@@ -1162,11 +1162,13 @@
 	}
 
 	/**
-	 * Returns the index of an element within its parent
+	 * Returns the index of an element within its parent for a selected set of
+	 * elements
 	 * @param  {HTMLElement} el
+	 * @param  {selector} selector
 	 * @return {number}
 	 */
-	function _index(el) {
+	function _index(el, selector) {
 		var index = 0;
 
 		if (!el || !el.parentNode) {
@@ -1174,7 +1176,8 @@
 		}
 
 		while (el && (el = el.previousElementSibling)) {
-			if (el.nodeName.toUpperCase() !== 'TEMPLATE') {
+			if (el.nodeName.toUpperCase() !== 'TEMPLATE'
+					&& _matches(el, selector)) {
 				index++;
 			}
 		}
@@ -1182,6 +1185,23 @@
 		return index;
 	}
 
+	function _matches(/**HTMLElement*/el, /**String*/selector) {
+		var matches,
+			i = 0;
+
+		if (el.matches) {
+			return el.matches(selector);
+		} else if (el.matchesSelector) {
+			return el.matchesSelector(selector);
+		} else {
+			matches = (el.document || el.ownerDocument).querySelectorAll(selector);
+			while (matches[i] && matches[i] !== el) {
+				i++;
+			}
+			return matches[i] ? true : false;
+		}
+	}
+
 	function _throttle(callback, ms) {
 		var args, _this;