Add unit test to project.

[TylerYang]

When the basic code is becoming more and more stable(not now, of course), it's the right time to add the Unit Tests.
Consider following test framework,

1. Mocha
2. Jasmine
3. Jest

And here is an interesting vote. According to the result. Seems most company choose the mocha. Not too much company would like to use Jest.

But what make me want to use Jest is, first of all, _FB_ invented it, and the next is, it is used to test react-* project. Such as _react-native_.

I used to use _Jasmine(For Angular project) and _Mocha(For NodeJS project), both of them are good enough. But _React_ + _ES6_ make everything different. Better to write some demo projects to try them out, for now I prefer to use _JEST_.