Basic framework for automated testing with travis CI

[Travis CI](https://travis-ci.org) is a pretty cool platform for running automated tests on github, and [Craig Citro](https://github.com/craigcitro) has been kind enough to integrate R with travis.  This means that any time someone sends in a PR, travis will automatically run R CMD CHECK and make sure their changes don't break anything.

I also setup a minimal testing infrastructure with [testthat](https://github.com/hadley/testthat) so we can add unit tests for the important R functions.

The tests take a long time to run, as R CMD CHECK builds all the vignettes AND runs all the code therein, but it's worth the wait to make sure all changes pass this important test.

Over the next few weeks, I'll try to add more unit tests to testthat()