Support for @-prefixed keywords in the each block helper

This adds the support for data keywords prefixed by `@` such as `@index`, `@key` in Handlebars' each block helper. Prior discussion about supporting `@index` at #112 

Note that support for `@first` and `@index` in object iteration was added in Handlebars in 1.2.0. https://github.com/wycats/handlebars.js/pull/661