Add kaOauthAjax call (so mobile web view can send authenticated requests without cookies)

Summary: The iOS web view doesn't set cookies, so we need to handle authentication via oauth. If the oauth library is included, we'll use that mechanism to make the API request; otherwise, this returns the $.ajax request that was used before.

Test Plan: Do some exercises while logged into the iOS & web apps; advancement works!

Reviewers: mgp, ben

Reviewed By: ben

Subscribers: marcos, alpert

Differential Revision: http://phabricator.khanacademy.org/D8300