Start to use accessor functions when working with session information.

* Add get/set/clear functionality for Session.
Session is now saved to localStorage to allow the user to reload the screen.

* Must use accessor functions to save data to Session
* Data stored in localStorage is automatically loaded into Session on startup.

fixes #310
Users must be signed in to see the settings and delete_account pages.
Notify the Session.channel of user status changes from within fxa-client.js

* Reduces the number of places this needs to be done.