Add unit tests

[wleoncio]

This package doesn't seem to contain a testing framework, so new code contributions are at risk of inadvertently breaking something. I, for one, really enjoy using the testthat package for that. If I have some spare time in the near future, I could help you set this up (or improve it, if someone else gets around to it faster).

[GuangchuangYu]

Thanks @wleoncio ! I am glad to see it happen.