Use promises instead of done() for async tests

[motiz88]

This uses Mocha's neat promise support and a bit of promise chaining to generate more meaningful test failures during development while keeping the code clean.

(The alternative would have been to put a try { ... } catch(e) { done(e); } around each deferred expect block, and that's just plain nasty)

I'll happily rebase #83 in this style if this PR is accepted.

[wheresrhys]

great - thanks