Traceur compatibility

What about this solution? :)

I tried it out and it works for
- Async `function(done) { ... done(); }`
- Promise `function() { ... return promise; }`
- Sync `function() { ... }`
- Generator `function*() { ... }`

I'm not sure if this can be done that cleanly without a promise lib. If you don't mind having RSVP as a dep I think this is a great solution.

~~Don't merge it yet, still want to try something out...~~
Can't think of a better solution. If you find one and don't merge this PR it's fine with me, too. I really only care about that it somehow becomes compatible with traceur. The other package `mocha-co` is compatible with traceur, but I don't like that it's a complete mocha fork.