Add more unit tests (maybe in a continuous integration workflow)

[theclue]

DIFFICULT: almost easy

facebook.S4 has a preliminary set of unit tests using testthat

This set should be enriched and code coverage should be increased as well.

A Travis file for automating this would definitively help!

HACKTOBERFEST: pull requests against this issue are elegible to rewards in the contexct of the Hactoberfest 2018

[llrs]

I used usethis::use_travis to add a the file needed for travis for ci, however maybe the test are skiped on travis because of the skip(...) lines it has. In addition, on my local machine the tests are failing, so I can't test new tests:

Error in x[[method]](...) : attempt to apply non-function
Calls: <Anonymous> ... <Anonymous> -> o_apply -> lapply -> FUN -> <Anonymous>