extend security extension by csrf token manager and csrf token modifier

[naucon]

In twig templates a csrf token can be set with {{ csrf_token('your_token_id') }}. The pull request adds this functionality through a modifier plugin {'your_token_id'|csrf_token}.

The functionality is required for using the symfony authentication with csrf protection. Therefore a csrf token with the id "authenticate" has to be set in the login form.

{'authenticate'|csrf_token}

Functionality was mainly ported from the form_extension (branch) to the security extension. Because the form extension branch wouldn't be finished soon and symfony itself handles the csrf functionality in a separate security component - independent from the form component.

I would be grateful if you could merge this PR to the master.

[naucon]

Test breaks because Symfony 2.1 do not have the Symfony\Component\Security\Csrf\CsrfToken class. Looks like it was introduced in Symfony 2.3. Befor 2.3 the CSRF functionality was part of the form component.

I try to fix this problem an keep compatibility.

[naucon]

sorry for a lot of try and error commits. I should have used a real php5.3 environment with a old phpunit version.