Add tests

[Schlipak]

This is like, super needed!! Unfortunately I've had quite some troubles with module imports in tests. Tried Jasmine, Mocha and Ava, not sure for which to go and how to configure the module resolution to work properly. Usually if I get the resolution to find the module I want to import, the module's own imports fail since they are configured to work with the current Babel + Webpack configuration.

[Schlipak]

See Webpack doc