One-liner to add a 'since' parameter to iter_all_repos 

I needed to be able to restart a fetch of all projects on github.  Since the rate limit is
5000/hr, restarting after 100,000 project records meant days of repeated data.  Since
Github's API has it, it was about one line to fix.

I added a test to test_iter_all_repos to test the parameter as well.