add support for will_paginate; remove Kaminari dependency

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?

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

A dummy paginated class is provided for the purpose of tests.
no need to boot a whole dummy Rails app in testing

We only test a single controller in isolation and we can get away with
just initializing a dummy Rails.application object and declaring routes.

This gets rid of the runtime dependency to "rails" in the gemspec, which
in the long run is bad practice for plugins to declare anyway.
remove rspec-rails development dependency

We basically didn't use any of the features from rspec-rails, and the
important request/response stuff comes from
ActionController::TestCase::Behavior. This allows us to drop the
"activemodel" dependency as well.
Merge pull request #1 from mislav/will_paginate

Add support for will_paginate