Folder fundamentals are working:

- Add folders as entries in entry collections.
  - Folders are models that have entry collections as contents
- Add, remove, edit entries within folders, including other folders
- Navigate back up (currently, using an item in the main menu - will fix)
- Folders, contents, etc, are retained across sessions.

Changed the approach slightly:
- contained EntriesCollection is FolderModel.contents
- the id of the EntriesCollection is FolderModel.get("contentsId")
- the contained EntriesCollection is fetched every time the FolderModel
  is fetched. This means we defer heavily to Crypton for caching /
  updating behavior, rather than depending on backbone for it.
- we still use one sync

What's missing:

- Distinct back button, to ascend from folder
- Folder edit - probably should be in the main menu
- Folder delete - probably also in the main menu
  - must prompt to confirm deletion of all (recursive) contents
- Ordering - arbitrary
  - could be just alphabetical by name  <= probably best
  - or folders first, then all entries, alphabetic within sections
- Item relocate
  - menu entry for each individual item, and for folder