Use the master request from the stack

I think this solves #84, though not sure if it will cause problems with ESI.  Can anyone confirm?

Always using the master request from the request stack means that we're only concerned about the locale of the controlling page.

we could check if the container has `request_stack` and if not, fall back to `request`. that way the bundle would continue to work with symfony 2.3 LTS for those people that did not see the problem you are experiencing. something like
```
$request = $this->container->has('request_stack') 
    ? $this->container->get('request_stack')->getMasterRequest()
    : $this->container->get('request')
;
```

eventually, it would be a lot more elegant if we would inject the dependencies into the service rather than pulling them from the container. with the request stack, this would be possible: http://symfony.com/blog/new-in-symfony-2-4-the-request-stack