Now that Journey has been integrated into ActionDispatch we can raise
the exception ActionController::UrlGenerationError directly rather than
raising the internal Journey::Router::RoutingError and then have
ActionDispatch::Routing::RouteSet#generate re-raise the exception.
Move the Journey code underneath the ActionDispatch namespace so
that we don't pollute the global namespace with names that may
be used for models.
Fixesrails/journey#49.