Added attribute function with testcases

This adds `attribute` function which was added in Twig 1.2. 

First off, I am aware that it may not require in Twig.js and we can access `{{ obj[method](params) }}`. The reason why it's added is to provide one to one mapping between Twig and Twig.js templates.

Secondly, There are use cases why people want to have to for example we run same templates on server and on client in different context using our PHP backend and Twig.js on client side.

Hope it'll get it merged! :)
Cheers!