Handle export authorization in the controller

The first thing the Controller does is check if the user is authorized to
export issues. If they are not logged in, it will direct them to a login page.

This modification removes the authorization check from the view logic and adds
a clean redirection back to the issue view.

Fixes GH-12.
Bump version to 0.3.1

Backwards-compatible patch release with one change:

  - When Anonymous users do not have permission to export tickets, they are
    cleanly redirected to the login page and then back to the issue view
    (GH-12).