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. I was also able to backport a lot of code back into pyactiveresource.

## Changes
updated to the new pyactiveresource
removed duplicate code that is in pyactiveresource
updated changelog

@dylanahsmith 
@girasquid