Mocha reporter to generate coverage report of istanbul instrumented code.

1. Install istanbul, mocha and mocha-istanbul
2. Instrument your code using istanbul
3. Make your tests to use the instrumented code (see COA library as an example)
4. Run mocha using mocha-istanbul as a reporter
5. Use the environment variable ISTANBUL_REPORTERS to specify a comma separated list of istanbul reports. By default ISTANBUL_REPORTERS=text-summary,html