Update Context::get to resolve @key and @index

Change to allow @key and @index to be resolved within Context::get calls. 

For Example to port this javascript helper
    Handlebars.registerHelper("ifEquals", function (name, value,options){
        if (name == value) {
            return options.fn(this)
        }; 
    });

Helpers that could accept @key and @index as parameters previously had to implemented like 

    $handleBars->addHelper("ifEquals", function ($template, $context, $args, $source){
        list($property, $value) = explode(" ", $args);
        if ($property == "@index"){
            $property = $context->lastIndex();
        } else if ($property == "@key") {
            $property = $context->lastKey();
        } else {
            $property = $context->get($value);     
        }
        if ($value == "@index"){
            $value = $context->lastIndex();
        } else if ($property == "@key") {
            $value = $context->lastKey();
        } else {
            $value = $context->get($value);     
        }
        if ($property == $value){
            return $template->render($context);
        }       
    });

Can Now be implemented as 

    $handleBars->addHelper("ifEquals", function ($template, $context, $args, $source){
        list($property, $value) = explode(" ", $args);
        if ($context->get($property) == $context->get($value)){
            return $template->render($context);
        }       
    });