Update template helper API

After using it for a while, I became unsatisfied by `browserid_button`. It was becoming a bit of a monster in terms of parameters, and the `user.is_authenticated` check didn't seem to work well in our own template, as it makes customizing that logic a bit harder.

So I split it out into two functions, `browserid_login` and `browserid_logout`. Now users can have greater control over where the login and logout links appear and what they look like, and the code on our side is simpler (we can avoid most of the sillyness of having partially-evaluated helpers). 

Other issues that were addressed:

- The login buttons all now use a single form on the page. This avoids some issues with the <form> element being put inside of an inline element. As a part of this, browserid_info is now a function instead of just a variable.

- Template helpers are in helpers.py now.

- Documentation has been updated, and most of the autodoc stuff has been moved into a separate API page.

- Remove the Verify view logic to support a custom redirect parameter and just use `next`.

I'm just terrible at this "small commits" thing.