fix bug in case object is not promise but have promise property

Hi,
I very love your code and use it in my production apps like https://github.com/backand/angularbknd.
It's very powerful stuff because it don't load any image and work only with pure CSS.

I think you have a small bug in line 46, when parameters is not a promise,
but have a property called $promise.
I think is not right to check property name is '$promiseThing'.

Thanks again for your code.
Ygal