Add unit tests

[gre]

we need unit tests to avoid regressions when adding features/refactoring (this happened a few times recently). We need to try https://facebook.github.io/jest/ (like used in React)

I think this should wait next major release (2.0.0).

[gre]

I think it's a tough call for gl-react, would probably see this on gl-react-dom instead of gl-react. For now, we will continue writing more "tests" in the examples app