Add 'Sign In' and 'Sign Out' buttons

The JSON API uses Basic Auth, so we don't need to get a token or
anything here.  We just store the credentials in
$rootScope.credentials, and we'll use those to authenticate any API
requests.  That's not preserved across page reloads though, so we may
want to cache this information in localStorage [1,2].

We're still settling on the UI for this functionality.  From the
Basecamp discussion [3]:

* Do we show the cart box before users are logged in?  Undecided (yes
  [4], no because it would confuse users at checkout time [5] and it's
  hard to store unauthenticated carts [6]).  In this commit I've left
  that alone.
* Are there other parts of the UI that we want to hide before users
  are logged in?  Favorites?  My Account?  (yes, hide those [4]).  In
  this commit I've hidden those for unauthenticated users.
* Where should the login button be, and what should it look like?  The
  login button could go where My Account button is [4].  The text
  should be 'Sign In' ([6], inferred from 'Sign Out' mentioned in
  [4]).  In this commit I've put it next to My Account.
* Where should the logout button be, and what should it look like?
  Undecided (hidden on the My Account page [4], or replacing 'Sign In'
  [6,7]).  In this commit I've put it in the same place as 'Sign In'.

Christina went into more detail about the landing page being designed
to educate new users about our drop-based workflow and get them to
sign up for a drop before they move on to loading up their cart [5].

Still todo:

* Replace the 'Azure in a Nutshell' page with something else for
  authenticated users.  Presumably folks who have logged in don't need
  to create another account ;).  Lots of websites use a custom
  per-user dashboard here.
* Validate the credentials in ok() before accepting them.  For
  example, we can hit /person to get a user's name for personalizing
  the view.

[1]: http://dev.w3.org/html5/webstorage/
[2]: https://developer.mozilla.org/en-US/docs/Web/Guide/API/DOM/Storage
[3]: https://basecamp.com/2690730/projects/6465667/messages/34404751
[4]: https://basecamp.com/2690730/projects/6465667/messages/34404751#comment_224275997
     Benjamin Brewer, 2014-11-28 12:26 PST
[5]: https://basecamp.com/2690730/projects/6465667/messages/34404751#comment_224312546
     Christina Shinn, 2014-11-28 19:05 PST
[6]: https://basecamp.com/2690730/projects/6465667/messages/34404751#comment_224348556
     W. Trevor King, 2014-11-29 12:48 PST
[7]: https://basecamp.com/2690730/projects/6465667/messages/34404751#comment_224276742
     Susan Booth, 2014-11-28 12:31 PST