Fix conflict between translator service in ZF2 and skeleton app

This fixes a conflict raised by #4391 and reported in zendframework/ZendSkeletonApplication#184 -- basically, the skeleton application was defining a "translator" factory already, which meant that with the changes on ZF2, we were creating a conflict for anybody upgrading their existing app to the latest master.

What this PR does is create a new "MvcTranslator" service, using the MVC's translator factory. This ensures the original translator factory continues to work, while allowing validators to work in a turn-key fashion.