Add Silex bridge (ServiceProvider)

[dirkluijk]

I would like to contribute by adding a ServiceProvider for Silex applications. It should register the Twig extension and expose the same configuration the Symfony2 bridge does. The provider would live in the Cocur\HumanDate\Bridge\Silex namespace.

If you agree on this, I can provide a PR.

[florianeckerstorfer]

Would be great if you could provide a PR.