Added support for @index and @key

I've implemented @index and @key support. A short description of how:

```Handlebars_Context``` gets an *index* and *key*-stack. ```Handlebars_template->_variables()``` checks if the requested variable is ``@key`` or ``@index``, and returns the value from the context. Because it is a stack, accessing @index in nested lists, and @key in nested objects work without a problem.

Hope you could pull this request, because my main reason for using Handlebars is actually the support for these variables.