Fix fs.exists function

[mattzeunert]

exists's callback signature is just (value) instead of (err, value) so normally if the file exists it would be interpreted as an error because the first argument is true.

The change checks if the asyncFunction can return an error, if it can't it triggers a success.

[mattzeunert]

Thanks Kris,

I've moved it to not modify Promise.js.

I haven't found a way to pass the arity to the conversion function without modifying Promise.js, so for now it assumes the function takes two arguments (a value and a callback).

[mik01aj]

Hey guys, fs.exits an important function. Why is this PR waiting to be merged for 18 months? Is no-one using promised-io anymore? This is totally bizzare.