Move test dataset to inst/extdata so it can be conveniently loaded.

The normal data loading from data in 'data' assumes that CSV files use a semi-colon delimiter, and so our data file does not load properly.  In addition, it is difficult to get the data dir to load in both non-installed and installed packages in a consistent way. By relocating our test dataset to inst/extdata, both test() and test_package() functions work.
Adding a tests dir that runs all of the testthat tests.

These tests will now be run by R CMD check, and so we can ensure that the package is tested properly before release.  They are also run by the Travis CI system, and so we can track the introduction of test regressions.