Add segment-literal notation for expressions

I've added support for segment-literal notation for variables names as in Handlebars.js. In other words one can use ```value.[some key]``` syntax.

For example:
```php
$engine = new \Handlebars\Handlebars();

echo $engine->render(
    '{{greetings.[hello world!]}}',
    array(
        'greetings' => array(
            "hello world!" => 'Hello from Handlebars'
        )
    )
);
```
The tests for such feature in handlebars.js are available here: http://jsfiddle.net/L7LWM/1/