oauth secret support in response

Hello,

This PR adds getter for `oauth_token_secret` both to `HWI\Bundle\OAuthBundle\Security\Core\Authentication\Token\OAuthToken` and `HWI\Bundle\OAuthBundle\OAuth\Response\UserResponseInterface` and it's descendants.

Use case is simple: store both `oauth_token` and `oauth_token_secret` to be used for offline twitter posts.