Python 3 compatibility and minor bug fix

Hi Zach, thanks for the great library. Here is a very quick Python 3 compatibility patch (I would've used six but I didn't know whether you would be down with adding it as a dependency, let me know). 

I also noticed that repr(moment.now()) results in an error. I changed the clone method to copy _formula as well which seems like the right thing to do but I haven't looked extensively at the library so I'm not sure.