Making Client and Index new-style python classes for easier extension

Greetings,

I was attempting to subclass Algolia's `Client` object in my own project, and found that it wasn't a new-style class. I changed `Client` and `Index` to be new-style and added a test that shows subclassing. All tests are passing, but I only tried with Python 2.7.3

Please let me know if you need me to massage this to get it merged.