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.

Sorry for the giant changeset, but the changes just sort've piled up as I went along. The removal of the browserid_info could maybe be split out, as could the test updates if anyone thinks that's worth it, but if you can just handle mega-PR you'd be my best friend. :D