Adding a Paginator to fix Kaminari & WillPaginate conflicts.

Alternatively we could use the solution active_admin uses (make the user define a new per_page_method for Kaminari) but that either puts more work on the user's end, or has the potential side effect of breaking other gems. This way isn't perfect (our page select html in views are rendered differently for will_paginate and kaminari) but it seems like a solid start that prevents any extra work for users of upmin-admin.