Make package tree-shakeable

I am using webpack to bundle my code. It can remove unused code by the process called tree-shaking, but it can only do it if the code is side effect free, and and ES6 module.

For this I propose to mark the package side effect free by including `"sideEffects": false` in the package.json.
Also convert the code to proper ES module syntax by only using import / export instad of `module.exports`, `export = ...`, etc.