Add travis integration

Add a travis file that handles the install and runs R CMD check skipping the unit tests and examples, since that would require authentication.  Feel free to ignore this PR if you feel that under such conditions CI testing and a CI badge isn't really appropriate.  Perhaps this will provide a basis for future testing though. For instance, the package won't be able to go back onto CRAN until `R CMD check` runs successfully, so ultimately tests will have to include skip clauses for everything that require authentication.  Once that's done, the `--no-tests` flags can be removed from the travis here.  

Sorry for all the commits, building this on travis turned out to be more annoying than I realized, particularly due to some strange behavior on travis / `install_github` ignoring my requests not to install from the suggests lists...