Calculator review

This is an exercise for practicing code review.

To create a review:

1. Fork this repo to your own GitHub account
2. On the page for the new repo, select the implement-calculator branch from the drop-down list and hit "New pull request"
3. Make sure the base fork and branch are yourname/CalculatorReview-JS master, and the head fork and branch are yourname/CalculatorReview-JS implement-calculator
4. Click "Create pull request"