add a Python generator for eBay Feeds

[matecsaj]

A large data file download follows a data request with eBay Feeds APIs. Currently, every library user needs to write code for this. That is a waste of labour!

Compare this with most API calls that return JSON data in the RESTful response. The library already handles them nicely.

Ideally, add a generator similar to what currently exists for all API calls that return "paged" data.

It's likely a "breaking change," so do it before moving to production/stable.

Does someone out there already have code to handle this that they could contribute to the project? Even a starting point that could be refactored into the library would be welcome.

[matecsaj]

This might help https://github.com/eBay/FeedSDK-Python.