Kaminari and will_paginate collision

I tried to integrate this gem to existing project that historically uses `will_paginate`. It turns out `will_paginate` takes precedence over `kaminari`. See the issue https://github.com/amatsuda/kaminari/issues/162. I took a look at both. Long story short `kaminari` appends class method `page` to every class inherited from AR::B, whereas `will_paginate` pollutes Relation, Association. I think there should be way not to pollute the whole AR but instead use a technique proposed in comments there. Also I think we don't need this line `require kaminari` because it injects the code to AR::B and user's applications may use different gem, instead I suggest inject `kaminari` stuff to `Feeder::Item` manually. WDYT?
Does this ensure backwards compatibility for those who are relying on `Feeder::Item.x.page` like I think it does? :smile: