add support for purge and refresh with symfony HttpCache

fix #146

provide a purge and a refresh handler
ideally, ``$this->options['allow_reload']`` would be an extension point: https://github.com/symfony/symfony/blob/2.7/src/Symfony/Component/HttpKernel/HttpCache/HttpCache.php#L326

we could also have an event in lookup and let the subscriber somehow tell whether the cache should be bypassed. this would not interrupt the normal request flow. but we would have a bit more logic in the HttpCache class