fetchAsync example no longer valid

Created by: blwinters

In Readme.md, the following function generates the error: Cannot invoke 'fetchInBackgroundWithBlock' with an argument list of type '((PFObject!, NSError!) -> Void)'.

func fetchAsync(object: PFObject) -> BFTask {
    var task = BFTaskCompletionSource()
    object.fetchInBackgroundWithBlock {
        (object: PFObject!, error: NSError!) -> Void in
        if error == nil {
            task.setResult(object)
        } else {
            task.setError(error)
        }
    }
    return task.task
}