Code coverage tool (Istanbul) added.

Hello,
As a part of our Node.js course we are studying unit testing and tools for measuring code coverage. So I picked your module and added code coverage tool called Istanbul to your module. See https://www.npmjs.org/package/istanbul

Istanbul creates coverage-folder where the results are placed. In coverage/lcov-report folder you can find index.html which provides visual presentation of the code coverage.

To run code coverage tool: `npm run-script coverage`