Add unit test framework

[rmccue]

We need to make sure this plugin is thoroughly tested. Step 1 is adding the unit test framework.

[tfrommen]

I would like to take care of this. :wave:

However, what PHP version do we want/have to agree on? WordPress Core still is fine with PHP 5.2.4. The current plugin codebase requires at least PHP 5.4. And when it comes to PHPUnit, we either use the completely outdated and not even maintained version 4.8 (which is incompatible with PHP 7, by the way), or use the old stable, which requires PHP 5.6 (uh oh!) and also works with PHP 7+.

Also, to make use of WordPress-specific mocking features, I would suggest to use Brain Monkey, which I like much much better than WP_Mock.

Any comments on this?

[killua99]

I might try to add some.

Altho I'll require some help with the travis connection if we use travis. Perhaps Circle CI ? idk