Support wrap for function object

[solsson]

Is there a reason why wrap only supports an object with functions as properties, or would it also be possible to take a single function as argument and wrap it?

[tim-kos]

It would only make wrap's code a little more complex, but otherwise I see no reason. I'd be happy to accept a PR for this. :)

[solsson]

Might do that. We ended up with http://caolan.github.io/async/docs.html#retry now. Let's see what will come out on top :)

[tim-kos]

Alright, closing this then as nobody else seems to want this for now.