Add ItemVersion vector to perseus

Summary:
Adds the ability to create a perseus item data version
vector from perseus, which will be subperseized into
webapp so that the server can know whether an item
is renderable by a given version of perseus

Test Plan: node node/echo-item-version.js

Reviewers: joel

Reviewed By: joel

Subscribers: alex

Differential Revision: http://phabricator.khanacademy.org/D10711