use qunit with grunt

I made please work with qunit via grunt on PhantomJS.
A part of that was:
* adding qunit to grunt
* unifying error messages between browsers / engines
* making the errors more verbose, so the user gets a better idea why something failed
* adding two more tests that also have path element being `undefined`

Please let me know if you merge the JSHint PR (https://github.com/wingify/please.js/pull/5) first, so I can make the needed merges after.