Merge pull request #7 from kbroman/master

Output of arxiv_search is now a data frame
Merge pull request #9 from kbroman/master

Avoid printing search_info attribute with result of arxiv_count
Merge pull request #11 from kbroman/master

Forgot @export for print.arxiv_count
README: Add links and change example
Merge branch 'master' of github.com:kbroman/aRxiv
README: fix typo; move libary call from install to use