Add function to check if variable is a circular object

I run into the problem to check if an object is a circular one. So i found a useful answer (+function) on stackoverflow which is maybe also helpful for Underscorejs project. 

The answer is this: http://stackoverflow.com/a/13619725

No sure if you can simply copy this function but if there is a reference to the originally one i think that should not be a problem. I adapted the code a little bit to fit with your code conventions.