Remove axios and change browser bundle name

[bidoubiwa]

This PR removes axios and replace it with fetch.

Browser bundle will use the native fetch api and cjs bundle will use the fetch pollyfill.

Other additions:

• umd format now work on both node AND browser 🙌 fixes: Use iife for browser and cjs for node #470
• cjs format has been removed
• Tests are now made on both node environment and dom environment.
• Watch mode using yarn watch to help ease the development
• babel to make the bundeling more efficient
• Cleaned the error handler who now has the body that MeiliSearch gives him or an informative body that explains any other fail

Structural changes:

• Http class is not a parent class to MeiliSearch anymore. Which now means that if you console log MeiliSearch you will not see the post, get, put, ... methods inside its object.

This PR introduces some breakings:

• index.deleteIndex() now returns undefined instead of an empty string
• A bad URL will now raise a bad URL type instead of a meiliSearchCommunicationError

[curquiza]

It's not mentioned in the PR description, but has the .browser.js extension been removed? I would say yes 🙂

A bad URL will now raise a bad URL type instead of a meiliSearchCommunicationError

What is a bad URL? A wrong formatted URL?

[bidoubiwa]

Yes it has been replaced with .umd.js!
A bad url is something that is not an url,
abc => bad url
http://google.com => good url