Improve tests and documentation of the Shopping API.

* The tests for the Shopping API (`tests/test_shopping.py`) now work, and actually test something.

* The Shopping API itself (`ebay/shopping.py`) has now relatively extensive docstrings.

* One function of the Finding API (`ebay/finding.py`) has now documentation too, but documenting all functions is too much work for now. 

* The three test files that I worked on, `tests/test_shopping.py`, `tests/test_finding.py`, `tests/test_alternative_config`, now use an alternative configuration file. 

* Some polishing.

I'll merge the changes in a week or so, if no one objects.