Small changes to help docs for arxiv_open and arxiv_search
arxiv_open: get rid of arxiv_max2open opt and use function arg, limit

 - For controlling the maximum number of abstracts to open in browser
Further work on vignette
Vignette: title had "R/simcross"; oops!
arxiv_search: sort_by arg to take "submitted", "updated", "relevance"

 - have user write just "submitted" and "updated" rather than
   "submittedDate" and "lastUpdatedDate"
 - add internal function recode_sortby
Call set_arxiv_time() _after_ the call to httr:POST
arxiv_search: reduce default for batchsize to 100
Add a bunch to vignette
Forgot to update .Rd file for arxiv_search with new default batchsize
vignette: Various small changes
Remove inst/doc/arxiv_api.html from package; doesn't seem necessary