updated to the new pyactiveresource

## Backstory
This PR bumps the version of pyactiveresource required for the shopify python api. This means that the default format internally will now be JSON, this shouldn't affect users of this lib as it is all internal. 

In shopify/base.rb we inherit from pyactiveresource base and overide several methods, many because we were waiting on patches to be accepted upstream since we now forked pyactiveresource I was also able to backport a lot of code upstream.

## Changes
updated to the new pyactiveresource
removed duplicate code that is in pyactiveresource
updated changelog

@dylanahsmith 
@girasquid