convert tests from tap to lab

@phated I'm not big fan of tap's syntax (to put it mildly) so was happy to see that you've proposed moving to lab/chai. I've took a stab at converting tests from tap => lab as since it proved to be a pretty easy exercise, here is the PR.

The only test I didn't convert is [this one](https://github.com/phated/async-done/blob/31d148890a686a69f93d13b208ebe0c9d6f58c04/test/callbacks.js#L34-L44) as I wasn't quite sure what it is trying to verify :-/ Happy to amend the PR if you could provide some guidance. 

Closes #12