Better unit tests

[biw]

👷 WORK IN PROGRESS 🚧

This PR adds more (and better) unit tests to Nuts. Basically download and updates will be correctly unit tested to avoid breaking changes when trying to contribute to the repository.

This PR will also add standard HTTP status codes to responses, and fix #56.

I was hoping to move code to ES6 (cleaner code, especially for classes) and use Babel before npm releases, but I can't find a solution to make it work correctly while supporting the Heroku Deploy button (I'm open to ideas!).

I'll release a new version 🚀 as soon as all endpoints are correctly tested, and once I'm sure merging #95 did not break Nuts for GitBook and other devs.

XOXO 💕

From GitbookIO/nuts#100

[biw]

Need to double-check that this solves GitbookIO/nuts#102 (comment)