Models and Backbone Helper of API Explorer

Summary: Rather simple backbone models with backbone mixin for react (taken from react examples).

Test Plan: Creating PublicApi collection and populating it creates a Backbone representation of our api

Reviewers: alpert, joel

Reviewed By: alpert

CC: ben

Maniphest Tasks: T1210

Differential Revision: http://phabricator.khanacademy.org/D3008