Fetch per_page from model by default

[JiriChara]

Both kaminari and will_paginate support per_page configuration on the model:

# kaminari
class User < ActiveRecord::Base
  paginates_per 50
end

# will_paginate
class User < ActiveRecord::Base
  self.per_page = 50
end

This PR is adding a functionality to api-pagination to use the values from model by default, so the per_page doesn't have to be same for all resources or overridden in controller.

def index
  @users = User.all
  paginate @users # per_page will be 50
end

Fixes #48

[davidcelis]

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