Add external unit tests

We'd want to use e.g. `testthat` to test "bad examples" or very basic things that are not well-suited to go into the examples section of function documentation.

Nevertheless, the useful tests should all go into the examples section. Even simple use cases are useful for people to see immediately, so they could run it and see the output. The examples section gets tested by Travis automatically as part of `R CMD check`.

See also: #24