Replace browserid_button with browserid_login and browserid_logout.

- Move template helpers to helpers.py and replace browserid_button
  with browserid_login and browserid_logout instead.
- Add login form to browserid_info to avoid issues with having a form 
  tag inside an inline element.
- Simplify Verify view redirect to look directly for the `next` 
  parameter.
- Update browserid.js to work with the new single-login form.
Update docs and changelog.
Make browserid_info a function to fix the form CSRF.

Django's CSRF protection is difficult to use outside of a view. When
browserid_info was being generated in the context processor, it couldn't
get the {% csrf_token %} tag to output a CSRF token.

To fix this, we change browserid_info to a function that executes when
the template is rendered. By that point, the Django CSRF plumbing has
sorted itself out and {% csrf_token %} now outputs a token correctly.
This also has the side benefit of making the API a bit more consistent.
Drop support for Django 1.3.
Review fixes, reenable Django 1.3.7 testing to see if it works.