Add code coverage reporting

[kadamwhite]

• http://gotwarlost.github.io/istanbul/ & https://github.com/gotwarlost/istanbul
• https://www.npmjs.com/package/mocha-istanbul
• https://github.com/taichi/grunt-istanbul

Example "coverage" task definitions:

• "test": "npm run lint && npm run test-prep && istanbul --config=test/.istanbul.yml cover _mocha -- -b -R spec test/index.js"
• istanbul cover node_modules/.bin/_mocha -- --reporter $(REPORTER) cheerio
• "coverage": "istanbul -- cover ./node_modules/.bin/nodeunit tests/unit", jshint