Asserts that a node.js-style asynchronous function taking a single callback
will call it without passing a truthy value as the first parameter.

```javascript#async:true
function mySuccessfulAsyncFunction(cb) {
    setTimeout(cb, 0);
}

return expect(mySuccessfulAsyncFunction, 'to call the callback without error');
```

In case of a failing expectation you get the following output:

```javascript#async:true
function myFailingAsyncFunction(cb) {
    setTimeout(function () {
        cb(new Error('Oh dear'));
    }, 0);
}

return expect(myFailingAsyncFunction, 'to call the callback without error');
```

```output
expected
function myFailingAsyncFunction(cb) {
    setTimeout(function () {
        cb(new Error('Oh dear'));
    }, 0);
}
to call the callback without error
  called the callback with: Error('Oh dear')
```

The parameters passed to the callback (excluding the falsy error) are also
provided as the value of the returned promise, so you can do further
assertions like this:

```javascript
function asyncFn(cb) {
    cb(null, 123, 456);
}
```

```javascript#async:true
return expect(asyncFn, 'to call the callback without error').then(function (args) {
    // args will be [123, 456];
});
```

Or using the Bluebird-specific `.spread` extension:

```javascript#async:true
return expect(asyncFn, 'to call the callback without error').spread(function (result1, result2) {
    // result1 will be 123
    // result2 will be 456
});
```