Grunt: test coverage with Istanbul and Coveralls

As in the jQuery Mobile repo, I'm using grunt-coveralls package
on NPM to send the coverage reports to Coveralls.io

I also did a small change on npmLoadTasks on Gruntfile.js

I have tested the same Coveralls integration in a repo of mine:
leobalter/DexterJS#2c27421a
Gruntfile: Whitespace