R script to grab tables from arXiv API user manual
grab_api_manual_tables: Skip the table on result elements

 - stuff like <title>, <author>, etc; doesn't seem important to users of
   the package.
tables from arxiv API user manual

 - Add to Makefile, to construct the data sets
 - Add .gitignore file with *.Rout
 - Revise inst/scripts/README.md
Simplify arxiv_cats colnames
Fix bug in script to grab tables from API user manual

 - wanted to drop the "id" row from query_prefixes, but the code didn't
   _really_ do so.
Add docs for the two datasets: query_prefixes, arxiv_cats