Implemented Symfony reverse proxy support for user context hash.

> Ref #26

Finally the PR :smile: 
This adds support for Symfony RP for user context hash feature.

For this to work, you need to:
* Make your `AppCache` class extend `FOS\HttpCacheBundle\HttpCache` instead of `Symfony\Bundle\FrameworkBundle\HttpCache\HttpCache`.
* Ensure `USER_HASH_ACCEPT_HEADER`, `USER_HASH_HEADER` and `USER_HASH_METHOD` constants match your configuration (cannot rely on semantic config here as we're way too early for that).
* Ensure `SESSION_NAME_PREFIX` matches your environment.

In any case, it is possible to redefine those constants to fit your configuration.

## TODO
* [x] Implement user context hash support for Symfony RP
* [ ] Clear documentation on how to set it up
* [ ] Make user context hash response vary on sessionId(s) only as Cookie may contain undesired info (e.g. analytics)
* [ ] Try to do some tests