Generate code coverage reports

[contolini]

This replaces the mochaTest grunt task with mocha_istanbul to track code coverage. Coverage reports are output to test/coverage and sent to coveralls.io. A coverage badge has been added to the README.

[ascott1]

ohhhhh, this is exciting

[mthibos]

@ascott1 What's a code coverage report? I'm excited too, just want to know what about! 😀

[stephanieosan]

Test coverage?

[contolini]

@stephanieosan Bingo. It's called code coverage instead of test coverage because you're covering the code in tests.

It's good practice to write automated tests that verify your code works as expected (and that it doesn't break as updates are made down the road). As a codebase grows, it can be hard to keep track of which parts of the application are covered by tests and which are not.

Code coverage tools tell you how much of your code is covered by tests. They generate reports that tell you "Hey! Function ABC and XYZ don't have any tests written for them." They'll also tell you the percentage of your code that is covered by tests. e.g. If you update the application and your code coverage drops from 100% to 85%, it means you failed to write proper tests for that new code.

[stephanieosan]

I love this!