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.  This is the approach taken to [port Django to python 3](https://docs.djangoproject.com/en/dev/topics/python3/).

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.
Wondered why this didn't need to be `six.iteritems(self.attributes)` but I see `sorted` takes any iterable, so it can handle both the 2.x list and the 3.x iterator produced by `items()` ... right?