Added compatibility for an array as a promise

Until now we could only except an object with a promise or an array of
of promises. What I need to achieve is using angular-busy with an array
**with** a promise property.

This commonly happens when using angular's `$resource` service and the
server responds with an array rather than an object.

```js
function Blog($resource) {
  return $resource.service('/blogs');
}

function BlogCtrl(Blog, $scope) {
  $scope.blogs = Blogs.query();
}
```

When we use syntax like this we get an array with a promise property. A
bit like doing this:

```js
$scope.blogs = [];
$scope.blogs.$promise = $q.defer().promise;
```