Notification polling

TODO: 

- [x] Add `NotificationProvider` instance for polling for new notifications
- [x] Add `Html5NotificationDispatcher` which will (optionally, depending on browser features) notify the browser that there is a new notification via the HTML5 Notification API
- [x] Add `AmbientNotificationDispatcher` which will notify using in-app notifications
- [x] Add "since" parameter support to `/api/notifications/list` action so that only notifications received since a given date/time are returned
- [x] Fix race condition with `App.Notification.loadNotifications()` by making it a callback for `App.User.loadCurrentUser()`
- [x] Create a facade for `jquery.noty` which will ease the transition to another notification library if necessary
- [x] Integrate "since" parameter support on the client side

Fixes #6