Tests and CI

[raymondben]

Testing has been very minimal so far!
Add testthat testing, including on a range of datasets and using all possible mirrors. Add CI via travis, appveyor, use covr, all the usual loot.

[kimnewzealand]

I created a couple of unit tests to get the ball rolling.

I noticed two things:

1. The new prompt in the interactive to onLoad option @daranzolin means I can run the tests manually using test() but halts the tests using check(). Just wondering how you suggest the tests select the options?
2. the qa_dataset() is returning class tbl_df/tbl/data.frame not yet as qa_dataset @raymondben so it is failing.

[kimnewzealand]

1. is no longer an issue, Check is now running

[raymondben]

Closing: travis (45ed645), appveyor (ed601fb), codecov (f759531) all enabled