NotebookManager API: rename *notebook_model methods to *notebook
Add method notebook_exists to NotebookManager.

It is called by IPython.html.notebook.NotebookHandler.get,
so it must be implemented by every notebook handler.
Move method increment_filename from FileNotebookManager to NotebookManager

The implementation requires only a minor change (call notebook_exists
rather than checking for file existence) to be universally usable
in other notebook managers.
Rearrange the NotebookManager methods for clarity

Methods are organized into three categories:
 - Methods that must be implemented in subclasses
 - Methods that have default implementations, but can
   be overridden if required.
 - Support methods meant to be called by methods in subclasses
Remove NotebookManager._notebook_dir_changed.

This method is never called.