hook up login + logout with commonplace via shared token (bug 970598)

(Related to https://github.com/mozilla/commonplace/pull/24): "Not sure if this is what we want, but this allows centralized login/logout so there is no disparity between who you are logged in as no matter which component of Marketplace you are on."

- Set token in localStorage when logging in. Clear token from localStorage when logging out.
- Have Marketplace login use ```accounts``` API rather than ```users.views:browserid_authenticate```.
- When logging in via ```accounts``` Login API view, also log into Django's auth.
- Add a ```accounts``` Logout API view that logs out from Django's auth.