Use custom routes proxy

I'm working on an app that uses a Rails Engine (Spree). In a template that I'm working in, I need Kaminari to compute the URL using `main_app.url_for`, not `self.url_for`. 

This pull request allows the routes_proxy to be passed explicitly like:

```ruby
paginate @orders, routes_proxy: main_app
```

`main_app` is an instance of `ActionDispatch::Routing::RoutesProxy`. For more information, see the [Routes section of the Rails Guide to Engines](http://guides.rubyonrails.org/engines.html#routes).