Add support for will_paginate

This monkey-patches WillPaginate::CollectionMethods to make it quack like
Kaminari::PageScopeMethods, so that the plugin supports both libraries.

There's no reason why Kaminari runtime dependency needs to be declared, as
api-pagination will operate on any object that responds to:
- current_page
- total_pages
- first_page?
- last_page?

Optionally, you may pull my additional 2 commits that eliminate almost 20 gems from development dependencies and remove the need for the dummy Rails app in testing.