Added code coverage

Integrated istanbul code coverage library into TiShadow project. Added -c option to run specs with code coverage and supports all istanbul reports.

The code will be instrumented before to built the zip file that will be send to the TiShadow app. When the Tishadow app finished to run all specs a coverage object will be sent to Tishadow server as a cover log then the coverage report will be created. 

Reports to generate are the same supported by istanbul. https://github.com/gotwarlost/istanbul

Note: To make JUnit + Coverage work it requires the pull request https://github.com/dbankier/TiShadow/pull/295. Without --junit-xml, coverage reports works fine.