Travis CI

The commit 261cb7d3c reminded me that we really need to enable Travis CI here (whoever has admin access please do it); I could have realized the problem before I merged #324

After I enabled it in my fork, I realized that we had got errors in R CMD check: https://travis-ci.org/yihui/shiny At the moment, I have no idea when these errors came in (did not run `git bisect`). The documentation problems seem easy to fix (@trestletech @jjallaire).