README: Notes -> Links; add link to arxiv.org
arxiv_count: just count results for a given search
arxiv_count: make result an integer
Check that search won't return huge number of results

 - Give error in this case, to avoid problems
 - Still need to implement the use of "batchsize"
arxiv_search: change defaults for start and end

 - Also, if end is NULL, get a total count and use that
Code to space out arxiv search requests

 - before call to POST, call delay_if_necessary()
 - sets environment variable aRxiv_time
is_too_many: revise defaults for start and end
fix tests of is_too_many

 - results changed since defaults for start and end changed
Add delays, if necessary, before calls to POST
function to count entries in search result (if it's a list)
Implement arxiv_search with batches of requests
Allow global options aRxiv_delay and aRxiv_toomany

 - look for these for default delay between POSTs and number of records
   that is too many to retrieve; otherwise, use defaults
arxiv_search: I think the batches are working now
Reduce delay between POSTs during tests

 - also remove delay arg from arxiv_search help info, since no longer
   being used.
.Rbuildignore: inst/doc/src/ -> inst/doc/src
Fix bug in get_arxiv_time; was returning NA
arxiv_count example: set delay to 1 sec to speed up test