Add python 3.x support

## Problem

Python 3.0 was released over 5 years ago, yet pyactiveresource hasn't supported it.  Most of the major packages now have python 3, including django.  We should avoid being one of those packages that are holding back python 3 adoption.

## Solution

Use the [six package](https://pythonhosted.org/six/) to make the code compatible with python 2.x and 3.x.

I've gotten the test suite to pass with python 3.3 and 3.4 without breaking the test suite for python 2.6 and 2.7.