Task: adds the qunitnode multi-task

it's really hard to choose the best name for anything. I've pushed the task as `qunitn` but I can rename it.

Suggestions:

- `qunitnode`
- `node-qunit`
- `nodeQUnit`
- `qunit` (actually I don't want this one because I want to plug this task in another project that should run tests with PhantomJS and Node environments. That might conflict, right?

Edit: the tests and methods were written after `grunt-contrib-qunit`.