test for per_page param on collection

The tests for the new ``per_page`` param are being implemented in the /entity/country resource
since that resource was specifically created as a testbed for collections containing sereval
pages.
implement per_page request param

The per_page request param should now work on all collection resources. Paging through the results
is still possible by using the link headers since they also contain the pagination info.

I hope this can stay like it is, but we might need to re-add a server-side upper boundary check for the per_page
number. I'm leaving that open for the moment since it hasn't been decided where that boundary should be.
Merge branch 'feature/pagination-per-page-param' into develop