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.

is Intl always present or only from 2.4 on? i remember there was a up and down on this. 
maybe we could explicitly require the intl component if we need this here, if you do the fix i propose above to support symfony 2.3 again.