Introduce TypeScript

[usirin]

• [x] replace babel with tsc
• [x] replace common-js require with es6 modules. -- there is a little bit issue here with default exports of npm dependencies, some might require to be imported as import * as foo from 'foo'
• [x] replace common-js module.exports with es6 module exports.
• [x] upgrade prettier to ^1.4.1 for TypeScript support.
• [x] use ts-node for running tests with mocha.
• [ ] add module declarations via .d.ts files
• [ ] replace all js files under /src folder with their ts equivalents.

This PR's main job is to replace babel with typescript's tsc compiler. As of version 1.8.0 TypeScript allows using JavaScript files via allowJs compiler option in tsconfig.json. Since TypeScript knows how to transpile ES6 code into ES5 code, this allows us to use TypeScript along side with JavaScript, providing a way for us to incrementally migrate to TypeScript.

Even though this changes how we write code, my aim is to keep the same public api 100%, once we merge this PR.

[usirin]

gonna close this one for now, I will send another PR fixing for unifying the export/import statements and get rid of requires for now.