f.cb() -> f.onComplete()

To go with f.onError() and f.onSuccess()—this is the only thing that stuck out at me in an otherwise perfect API :grinning: It's a bit longer, but in a good, more noticeable way. And "complete" follows (among others) the jQuery convention.

I also included backwards compatibility and updated the docs & tests. (The tests all pass except for "ff #exceptions() should be propagated", which I think is unrelated.)

:v: