Implement ajax login. Fix bug where task duration not live-updating.

* Animate collapse of welcome header element on login.

* Make "home#index" action add a cookie with user info if user is logged in.
  This info is then put into angular $rootScope in the app's "run" block.

* Remove "welcome" action from "home" controller.

* Add controller to "task" directive which now updates displayed duration
  properly when resizing.

* Protect task controller actions with new "ajax_auth_user!" method which will
  return null JSON with a 401 status if user is not logged in.

* Fix typo in sessions_controller.rb.

* Make "logout" link in navbar ajaxy.

* Always render the "welcome" partial in index.html now.  This is part of a
  larger effort to make the app only serve one static html document where all
  changes are made in the client logic.

* Subclass devise "sessions" controller for ajax login.

* Modify devise initializer to allow ajax login.