Fix dynamic route add

[hellorachid]

when a user doesn't create routes and he wanna add dynamic route subroute throw error because the varibale routes is undifined
example :
var browserModule = new Backbone.SubRoute("browser",{createTrailingSlashRoutes: true});
browserModule.route('browse',function(){console.log('browse');})

[geekdave]

Thanks for the fix, @hellorachid ! I've merged it in a separate PR, since I needed to add a unit test for this case in order to get code coverage to pass.