Add support for reading multiple pages of github responses.

Summary:
A github API response will only give 30 repositories at a time; you're
supposed to fetch the rest via the url in the 'Link' header.  Who
knew?  Alpert, apparently.

So now I do that.

Simplify the check of the Link: header.

Suggested by alpert.

Test Plan:
Locally ran
   ./update_phabricator_repositories.py -v ~/backups/
and verified it said
   Fetching url https://api.github.com/orgs/Khan/repos
   Fetching url https://api.github.com/orgs/Khan/repos?page=2
and added 18 new github repos to my backups.

Reviewers: alpert, cbhl

Reviewed By: cbhl

Differential Revision: http://phabricator.khanacademy.org/D2009