Use dedicated settings file for building documentation.

Also fix up some paths in the API docs, remove `sanity_checks` from the 
documentation (it's not for use outside the library, mostly), and add
`MockVerifier` to the django_browserid init for importing.
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.
Fix #196: Add support for BrowserID in the admin interface.