timeout args to arxiv_search and _count

 - Also, an environment variable to allow timeout to just print a
   message rather than give an error.
timeout amount from environment variable rather than argument

  - default is 30 seconds
Update .Rd files to roxygen 4.1.0
when on CRAN, skip tests that connect to arXiv
Can't use tryCatch() in arxiv_count/search because it doesn't stop errors within vignettes
another test to prevent on CRAN
Try to avoid errors in building vignette when on CRAN (with arXiv connection issues)
Fix bug in arxiv_search in case limit=NULL
suppress messages for one test
Travis: Add environment variable NOT_CRAN=true
Appveyor: Add environment variable NOT_CRAN=true
another small test revision to avoiding running on CRAN
Merge pull request #24 from kbroman/master

Attempting to deal with arXiv connection issues and CRAN