I forgot to include the DOIs in output
Two more things re inclusion of DOI in output
Include better examples
In output, use "submitted" rather than "published"

 - API output says "published" but "submitted" is more meaningful (and
   is what actually appears on the arXiv abstract pages)
In test-search, speed up by using shorter delay
arxiv_open: increase default max no. abstracts to open to 20

 - using a 3 sec delay, so might as well open 20 if user is willing to
   wait a minute
arxiv_search: more detail on sort parameters

 - also change default to sort by submission date
 - key detail: sorting doesn't seem to work if id_list is provided; in
   that case, records are sorted by appearance in id_list
Add tests of sort_by and sort_order in arxiv_search