Add store.js

Add store.js, a simple wrapper around the
localStorage functionality.
Save session state to localStorage

Use store.js (which uses localStorage, with fallbacks)
to save our session data to disk, and resume previous
sessions when the site is revisited (from the same
computer).
Refactor: Add header to the top of all views

Move the common header into its own partial, and
include it using ng-include.
Refactor: Make header controller

Take the duplicate getUsername function out
of the various partials and put it in one
HeaderCtrl.
Header: Show 'log in' or 'log out', not both

When we're logged in, no need to show a link for
logging in, etc. This was already in the Contribute
controller, so I just moved it to the header.
Add a TopLevelCtrl that is the parent of all

Add a top-level controller that is associated
with the top-level container, and hence a parent
for all controllers in our app.

Put things that are used across the app with in,
like whether we are logged in.
Refactor: Move some log-in func into TopLevel

The HelloCtrl was doing some work to save the
logged-in user into the session, and this sort
of thing should be in the top-level controller.