Add test coverage report

Use `jscoverage` tool and Mocha to built test coverage report.

Install jscoverage: https://github.com/visionmedia/node-jscoverage#installation
Run:

```
npm run-script test-cov
open ./test/test-coverage.html
```

Get something like this:

![screen 20shot 202013-12-20 20at 2017 47 11](https://f.cloud.github.com/assets/872004/1790683/58a2be2a-697d-11e3-8763-bc8897601ca4.png)