Document regression tests

[KirstieJane]

We've implemented Travis continuous integration (wooo 🙌) and added some regression tests in #21 but we need to actually document what these tests are doing.

Imagine you're explaining them to me....who has never done any testing before 😬

THANK YOU!

[KirstieJane]

The documentation that's inside the code now is really great. It would be nice to have an easy to read overview somewhere, but not super high priority...

[Islast]

I thought I would move some of the discussion from issue #8 over here

In the longer run, a little document explaining our (the project's) philosophy on testing would be really great. Something like 2-3 paragraphs and some links to useful resources and the specific tools you're using would be really helpful 😄

[Islast]

N.B. following pull request #31 all unittest tests in modules with names ending in "_tests" will be run by Travis