Add browserid auth backend
Implement browserid login process
Allow users to log out regardless of auth backend

Logging out of BrowserID requires calling navigator.id.logout()
client-side, meaning that the "logout" link does a preventDefault to
stop going to the logout page until that function finishes. Problem is,
that redirection to the logout page only occurs if a user is, in fact,
authenticated with BrowserID. We're using the regular django auth
backend as well right now, so it's not necessarily true that a user will
be logged in with BrowserID.

This adds a variable specifying whether a user is logged in with the
BrowserID auth backend, making the logout work normally if not.
Handle invalid logins more gracefully

I'm all about grace here
Improve project roster viewing, editing
Allow project editors to invite users (by email)
Create accounts for users who have been invited
Clarify login page.

* Add explanation about invitations

* Add link to Mozilla Persona about page

* Visually, place the persona login button above username/password.

Users should prefer to use Persona. However, we should still make a
"reset password" fn so that we don't rely completely on the Persona
servers being up. That would make sense with Django 1.5 when we can
replace the User model-- users' names should just be their email, maybe.
Merge pull request #129 from editorsnotes/auth-tweaks

Implement BrowserID & invitations