Add testing environment

[bcolucci]

In order to be able to test features and avoid regressions, it seems to me important to have a testing environment. That's why I propose these little changes.

• Moving from babel 2015 preset to recommended env preset.
• Creating testing environment with jest and jsdom.
• Adding a first test for the init function.

[iztsv]

Perfect suggestion! I'll help with this after merge 😃