A script that makes sure all KA employees have access to all github repos.

Summary:
It does this by making sure 'dev-fulltime' and 'interns' are members
of every Khan repo.

Test Plan:
Created ~/github.team_token with the contents of
github_role_account_team_token in secrets.py, then ran
   ./update_github_teams.py -v
and saw it say a bunch of stuff like:
    Adding the dev-fulltime team to Khan/gflags
    PUT-ing url https://api.github.com/teams/874617/repos/Khan/gflags
    Adding the interns team to Khan/gflags
    PUT-ing url https://api.github.com/teams/874619/repos/Khan/gflags

Then I ran it again and it had no output at all.

Then I ran
   curl -u <team_token>:x-oauth-basic https://api.github.com/teams/874619/repos\?per_page=100 | less
and saw an entry for Khan/gflags.

Reviewers: alpert

Reviewed By: alpert

Differential Revision: http://phabricator.khanacademy.org/D13116