Add auth_token to users
Add before_save filter to User model to ensure auth_token is set

* Create an auth token as a secure randomized hex
Add token authentication
Add specs for SnapshotsController

* Don't worry about show action because it's going to go away once we
  hook up the super cool pusher events :)
Install pusher dependency
Add #notify_new_snapshot to send Pusher event
Send pusher notification when new snapshot is added
Add pusher js to app layout
Simplify user index markup
Update user JS to use pusher events!