Improve paging

Hi Gavin, sorry for the wait, I've splitted the commits related to paging to a single pull request. I've also upgraded to the last version of kaminari

There may be one more refactoring (which I haven't done yet). Currently the page_size is set in restpack_serializer config, but since it now uses kaminari's max_per_page, it'd make sense removing the page_size config and instead using kaminari's config (default_per_page) also for this. What do you think?