modified deepCopy to prevent infinite loops in cyclic references

Here's the pull request for issue 1 (https://github.com/mbrowne/simpleoo.js/issues/1).  It seems to work fine with the "visited objects" array.  This adds space and time complexity for large objects, but considering what people actually would use this for, I think it should be fine...  

Check out my code and make sure it looks how you want it.  I also added a test in test.html.