Folders are working

(Note - ``npm install`` necessary, to get a missing piece for deferred promise objects.)

- Folders can be added, persist, like other items
  - Folders are models that have entry collections as contents
  - contained EntriesCollection is ``FolderModel.contents``
  - the id of the EntriesCollection is ``FolderModel.get("contentsId")``
- Navigate into a folder by tapping on it
  - there, you can add any kind of entries, including other folders
  - use a back button to ascend back  (button currently inside the main menu, should be separate)
- The contained EntriesCollection is fetched every time the FolderModel
  is fetched. This means we defer heavily to Crypton for caching /
  updating behaviors, rather than depending on backbone for it.
- we still have just one sync routine, in backbone.crypton.js
  - we may want to rethink that.  including unravelling ``.modelId`` versus ``.container``
    versus (new) ``FolderModel.contents`` and ``.contentsId``

What's missing:

- Distinct back button, to ascend from folder
  - I think we should show both the back button and menu button on
    every view *except* the top-level entries folder
  - where we should just show the menu button
  - the order should be menu button then back button
  - I would have implemented this, but there are choices to make. And some code churn.
- Folder name in screen title
- Folder (name) edit - in main menu, for every folder view except top level
- Folder delete - also in main menu, also for every folder except top level
  - prompts to confirm deletion of all (recursive) contents
- Ordering - currently too arbitrary
  - probably best: all items lexically sorted by name
  - but could be folders first, then all entries, each section lexically sorted
- Item relocate
  - relocate menu entry for any item, including folders, except top level
  - it presents a navigation widget, by which user can nav to new location for the item
  - starts from container of current item