Implemented Symfony reverse proxy support for user context hash.

> Ref #26
> Relates to https://github.com/ezsystems/ezpublish-kernel/pull/1025

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
* [x] Clear documentation on how to set it up
* [x] Make user context hash response vary on sessionId(s) only as Cookie may contain undesired info (e.g. analytics)
* [x] Try to do some tests
Well, I don't agree on this. Relying on whole cookie string *will* do unexpected things as you'll have analytics and other unwanted stuff, which will result to unreliable caching of user hash, and thus heavy requests going straight to the kernel for each request.
I'd prefer having a dedicated method for cleaning cookies as you suggested, which makes it possible not to do so, but that would be the user's choice.

We've been through this with eZ Publish, people complaining that the user hash wasn't properly cached, so applying lessons learned :wink: