Add configuration for logout handler

Currently, setting `user_context: enabled` will throw an exception if no proxy client is configured because of the logout handler, which requires the client. This PR solves that by adding a `user_context.logout_handler.enabled` config option, which defaults to auto, so won't be available without a proxy client.
> for me it would make sense to say i don't want to actively invalidate the cache.

True, but you can do that already by *not* [adding the logout handler to your Symfony firewall](http://foshttpcachebundle.readthedocs.org/en/latest/reference/configuration/user-context.html#context-hash-route). That’s what I meant: you don’t need to set `logout_handler` to `false` for that in the bundle config.