Add markdown doc with notes on arXiv API
Fix example http get; add example parsing XML
Remove XML/XML2R example from arxiv_api.md
Initial stab at arxiv_search; just returns the xml as a list
Remove "plans" section from arxiv_api doc
Add ToDo list in inst/

 - Add to Makefile, to convert it from .md to .html
Remove errant comma from description
README: remove "not installable" comment
README: more details on installation and basic usage
pull out xml -> list code as separate function
arxiv_search checks for arxiv errors; add tests