0.2.8 hook up login + logout with zamboni (bug 970598)

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.

- On initialization of the user module, look for the shared token if it was set from Zamboni (or from other commonplace sites).
- On logout, send a ping to Zamboni to ```django.contrib.auth.logout``` everywhere.