Twig: multiple fixes + new function url()

Some of the functions that use the DependencyInjector in the twig "bundle" did not work for me, i was getting this error:

    Using $this when not in object context in /[..]/app/incubator/Mvc/View/Engine/Twig.php

The issue was the use of $this inside the closure passed to Twig_SimpleFunction()

I also added an "url" function that is already available to the volt engine.