Add an API version to Perseus

Summary:
Added an API and Item/JSON version in src/version.json
Added the API major version to perseus build files created via subperseus
Removed operations.sh, moving its functionality directly into the
Makefile (which has access to the necessary version number)

Test Plan:
ran `make subperseus`
verified that the output in webapp was what I wanted:

    Untracked files:
      (use "git add <file>..." to include in what will be committed)

        javascript/perseus-package/perseus-0.js
        stylesheets/exercise-content-package/perseus-0.css

Reviewers: alex

Reviewed By: alex

Subscribers: eater

Differential Revision: http://phabricator.khanacademy.org/D9326