Fix #195: Add support for Python 3.2 and 3.3 and enable tests on them.

Thanks to @senden9 for most of these fixes.
Use Django's version of six and drop Django 1.3 support.

Django 1.3 lacks a decorator we want to use for Python 3 support, and
given that 1.3 is out-of-date and we already have a few workarounds in
place to keep it working, it's time to drop it. This includes removing
the workarounds I know of to clean up the code a bit.
Fix #197: Use AJAX for submitting assertions instead of a hidden form.

This is a large change, and breaks backwards compatibility in a lot of
ways, including new JSON responses from Verify, a new JavaScript API,
and moved URLs.

- Change Verify to return JSON.
- Remove `next` parameter from Verify, since we no longer need to
  redirect users.
- Fix #198: Replace browserid_info helper with Info view. This lets us 
  retrieve the CSRF token without relying on the helper.
- Update the JavaScript API to a Promise-like interface using jQuery
  Deferreds, making the entire API async now that we're using AJAX.
- Add Logout view so logout can be done via AJAX as well.
- Add `next` parameter to browserid_logout helper.
- Fix a few broken tests that weren't asserting anything.
- Move sanity_check tests to the right test file.
- Remove pyquery dependency.
- Add tests for behavior that wasn't being tested.