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.
I was actually getting circular problems from trying to keep these as relative imports, and starting applying this to a few places to resolve it.  The specific case where I encountered the circular import problem was between products and collections which referenced each other.