Added support for @first and @last in each.

This patch adds support for `@first` and `@last` when iterating with `each`. This is similar to [issue 486 in Handlebars.js](https://github.com/wycats/handlebars.js/issues/483) (implemented in version 1.1.0).

It is useful, for example, to write a comma-separated list without a comma after the last item.