Add support for Expressions within templates

This adds support for using symfony Expressions. 

This class is pretty much a copy of https://github.com/symfony/symfony/blob/master/src/Symfony/Bridge/Twig/Extension/ExpressionExtension.php

This allows you to use expressions with the security extension http://symfony.com/doc/current/book/security.html#book-security-template-expression
```smarty
{if '"ROLE_ADMIN" in roles or (user and user.isSuperAdmin())'|expression|is_granted }
Hi Admin!
{/if}
```