upgrade test suite

heya, thanks for giving the talk in London recently and proving all the free :beers: 

I've been flirting with the idea of contributing to this project as I can see a genuine benefit, we all know npm is full of cowboys ;)

IMHO the best place to start is with a solid test suite; the current assert only test suite is lacking in pretty reporters and doesn't even check if `auditPackage` calls the cb!

so anyhoo, I took the liberty of re-factoring your test suite to use `tape`, which also works in the browser and is nice and modular, it produces computer readable output as-well as offering the ability to pipe that output to everyone's hipster-cool reporter of choice.

-P