sinon.format explodes when formatting null-values

When trying to format null-values (such as generating the message for `stub(null)`), it parses null into `sinon.format`. Unfortunately, this failed horribly.

This was happening all the time in my sinon-chai enabled tests, as sinon-chai generates the message even when the tests are green (pure sinon only do it for red tests).

I also took the liberty to add a test-script to `package.json`, as it makes running the tests on node a lot more transparent and easy (`npm test` rocks :D).