Put examples in \donttest{} to avoid problems on CRAN
Function to test connection to arXiv; skip tests if can't connect
testthat.R: aRxiv -> arXiv
tests: error() -> stop()
vignette: stop with clearer error if no connection to arXiv
Fix typo in comment in vignette
Merge pull request #19 from kbroman/master

Attempt to avoid problems on CRAN if no arXiv connection