Grunt 4.0.0rc7 compatibility and Cobertura coverage report

I needed this plugin for setting up a CI environment for one of my JavaScript projects. Since the original repo didn't have any Grunt 4.0 compatible code I forked this repository. However, I found out that the plugin was not able to run with Grunt 4.0.0rc7 due to some minor incompatibilities.

Furthermore, I've also added a new feature: Cobertura report generation.