Add support for yarn package manager.

[JimiC]

There are several advantages for us to use Yarn.

1. Faster dependencies installation (speeds up the CI process noticeably).
2. Dependencies locking. No more different dependencies between machines (as last seen with CI).
3. Dependencies security.

Installation: https://yarnpkg.com/en/docs/install

Migrating to Yarn
npm install => yarn install
npm update => yarn upgrade

[jens1o]

As long as we can stil publish it via npm, it's ok for me.

[JimiC]

@jens1o Yarn doesn't replace npm. Consider it as an improved npm.