arxiv_api.md: add note that you can search for a range of dates
Revise arxiv_api.html with change in arxiv_api.md
A bunch of little functions to start cleaning up the records
Bug in clean_authors: "authors" -> "author"
Split out some of parse.R into clean.R
Add search/count example for range of dates
Delay POSTs if necessary in test-arxiv_errors
Another delay-if-necessary in test-arxiv_errors
clean_links: "" if missing

 - also, strip trailing semicolons if necessary; saw that in one
   instance with multiple DOI links
Add tests of clean_authors and clean_links
Add tests of clean_categories
clean_record() for converting record list -> simple vector

 - also added tests

 - helper function get_key: made version for plain list
   as well as the version for a list of lists, which is now called
   get_key_ll
listresult2df(): convert list of results to data.frame

 - also tests

 - also revised clean_record to have separator arg
separator arg -> sep (in clean.R and parse.R)

 - separator seemed more clear, but sep is more standard in R
   (e.g., in paste)
listresult2df: need to fix rownames of result
arxiv_search: output as data frame; make this version 0.2

 - include argument output_format that can be data.frame or list, but
   data.frame is the default

 - result includes an attribute "search_info" with search
   parameters (and the time at which the search was completed)
arxiv_count: include attribute with search parameters
Add blank line in example for arxiv_count
Include time zone in search_info attr

 - attribute to results of arxiv_search() and arxiv_count()
For missing affiliations, use "" rather than like "|||"

 - Was pasting affiliations from multiple authors; if all missing, we
   end up with a string with one fewer vertical bars than there are
   authors. Better to just make it an empty string.