Add oldindex property to sortupdate event data

knowing the old index is a useful feature for using this in an MVVC application, since the underlying array can be spliced instead of having to be rewritten.

AngularJS example:

```javascript
function SortableCTRL($scope) {
	$scope.sortableArray = ['One', 'Two', 'Three'];
	
	$scope.sortupdate = function(e, data) {
		var start = data.oldindex,
			end   = data.item.index();
		
		$scope.sortableArray.splice(end, 0,
			$scope.sortableArray.splice(start, 1)[0]);
		
		$scope.$apply();
	};
	
	setTimeout(function() {
		$('#sortable')
			.sortable()
			.on('sortupdate', $scope.sortupdate);
	});
}```
during ‘destroy’, remove event handlers added with ‘.on('sortupdate', …)’
Merge branch 'patch-1'
Merge https://github.com/flying-sheep/html5sortable
Update README