Add in gae_util.

Summary:
This allows scripts that rely on appengine SDK imports to be able to
programmatically fix the system paths to include them. This avoids
having to muck around with more global PYTHONPATH's that aren't well
documented (though the setup instructions for any given machine that
needs to run these scripts will still require downloading appengine
manually, since Google doesn't publish that as a Python package)

Test Plan: Ran gae_download.py manually and had the imports work without having me to modify any of my own global PYTHONPATH values

Reviewers: jace, yunfangjuan

Reviewed By: yunfangjuan

Differential Revision: http://phabricator.khanacademy.org/D49
Allow GAE installation's root dir to be a symlink

Test Plan:
gae_util.fix_sys_path() now works for me on a non-standard homebrew
installation. This is the same logic the webapp project uses, so fingers
crossed that it works for everyone else.