JS code as npm package and write unit tests

[giorgi-ghviniashvili]

It is a good practice to have JS codebase as npm package:

• easily make it as a single bundle (build)
• write unit tests
• make whole JS codebase as a es6 module

[giorgi-ghviniashvili]

@jhofman @chisingh

• I made the whole JS codebase a npm package.
• bundled code using rollup.js.
• wrote some tests in test/test.js file.