Implement unit testing

[oneillkza]

Right now, we have sort-of testing in the form of the examples that get run on package build. This at least tests that things don't completely crash. However, it might be a good idea to have proper unit testing, to ensure that functions behave in an expected way.

The testthat framework would probably be a good one to use for this.

https://cran.r-project.org/web/packages/testthat/index.html

I don't know when I'd be able to get to this, but I'd be happy to set up the Travis build to hook into it.