Remove dependence on Q and return the same type of promise that was passed in

[ForbesLindesay]

I've wanted this module so many times. Thanks so much for actually writing it.

I'd like to use this as part of the real-time rendering feature of regexplained.co.uk once I finish getting it to work with web-workers, but I'd rather use a more bare bones promise implementation (and I'm biased towards my own).

I also love the idea of making utilities that are promise library agnostic, so here goes.

[domenic]

Oh, wow, this is extremely clever. I thought I'd need to wait for Q to get a promise constructor---see #1---but this is even better. Thanks so much!

[ForbesLindesay]

I wish I could find a similar sollution for https://github.com/ForbesLindesay/throat