Bookish only

- Added a single `Backbone.Collection` that stores `ALL_CONTENT` loaded in the editor so far
- Added Drag-and-Drop support for books and folders
- updated to work with Marionette 1.0
- new content gets an `id` so it can be added to a book before saving
- Book editing is implemented as a tree of `Backbone.Model` (and `Marionette.CompositeView`) instead of a string of JSON (and recursive Handlebars templates)
- Cleaned up the `install-libs.sh` file