Generate code coverage reports based on the node-qunit tests with
istanbul.
Changes:
* Update README with npm run coverage
* Add .istanbul.yml to configure istanbul
* Add npm script "coverage" that runs istanbul and generates the reports
Example report: http://popups-coverage.surge.sh/lcov-report/index.html
Change-Id: I9be8c04b858a3ce6f4e29af2685b79253e3b4dca
