Add configuration option items_per_page for pagination

Allows a user to override the default of 30 items per page with a global config option.

For example in config/initializers/upmin_admin.rb add:

Upmin.configure do |config|
  config.items_per_page = 10
end
Option to set items per page, on a per model basis, v2

This builds on the previous PR for a global override of the number of items per page, and allows it to be provided on a per-model basis.

For example in app/upmin/models/admin_shipment.rb:

    class AdminShipment < Upmin::Model
      items_per_page 20
    end

If not specified, or no custom class exisits, then either the global override `config.items_per_page` or the builtin default will be used.
Better tests for Upmin.configuration.items_per_page and Model.items_per_page

Previous test was basic, and didn't ensure that overrides cascaded correctly.
Merge pull request #83 from mbrookes/items_per_model_v2

Option to set items per page, on a per model basis, v2