Set the executable bit on runnable SDK scripts

Summary:
This lets us set up symlinks in webapp/tools that can run as, e.g.,
tools/appcfg.py.

Test Plan: ./appcfg.py now works.

Reviewers: dylan

Reviewed By: dylan

Subscribers: dylan, csilvers

Differential Revision: http://phabricator.khanacademy.org/D9865