Add python 3.x compatibility

@pickle27 & @costford for review

Depends on https://github.com/Shopify/pyactiveresource/pull/9

Uses the six module to make the source code python 2 and 3 compatible.

Tested against python 3.4.1, using `export PYTHONPATH=../pyactiveresource` use the branch of pyactiveresource with python 3 support.  The test suite passed locally, and I was able to use `bin/shopify_api.py console` to list, show, create, update and destroy products.
The input is bytes, which has no encoding, and decode is passed an explicit encoding to convert it to a unicode string.  The response is coming from shopify, so it should always be UTF-8, however, it would be more correct to look at the HTTP Content-Type header to see if it specifies the charset.