Use Istanbul for code coverage

* Replace grunt-contrib-qunit with grunt-qunit-istanbul (special fork)

Added `grunt qunit:cov` to test in debug mode and generate reports, and `grunt
qunit:all` to test in normal mode without coverage.

Reports are spit at tests/report/index.html

Change-Id: Ib0d8dfc72f86ce3878f5d0b8943e1a38bf7b3703
Merge "Use Istanbul for code coverage"