Add test scripts

A couple of changes:

* Use npm scripts for testing (inspired by substack's [task automation with npm run](http://substack.net/task_automation_with_npm_run))

* mocha doesn't need to be installed globally, since it's already in the PATH (npm magic)

I believe this is easier, since `npm test` is a common thing.

By the way: All tasks could be automated using npm. Grunt wouldn't be needed anymore (I'm more than happy to do that).
No, npm does some PATH magic.