Added a sponsor_links default setting

This default setting assumes that sponsors will be a list of links.
Should preserve existing functionality for the Chicago legistar scraper
while allowing extended functionality.
Added test for getting Philly sponsors and also adapted existing tests to use DEFAULT_CONFIG

Two changes for this commit -
- Added test coverage for grabbing philadelphia sponsors
- Converted existing tests to use Config and DEFAULT_CONFIG - otherwise
all tests fail since they do not set sponsor_links