Added support for @index in #each-helper when used on lists.

You're welcome. Thanks for the quick merge!

I've now added support for @index when iterating over lists using ```#each```. When you use ```#each``` on an object, you can use ```@key``` to access the key, and when you iterate over a list, you can use ```@index``` to get the index. Although it is a bit annoying to think about it when you are writing the template, it is consistent with the documentation and Handlebars.js.