Support a List of Saved Notebooks

[switham]

There should be a directory, in a standard place, with a list of saved notebooks. Ideally, notebooks should be auto-saved.

On a developer's computer, it would be somewhere under the home directory. On Android, under /mnt, but I'm not sure exactly. Maybe somewhere related to the /mnt/SL4A directory.

When pyinthephone first comes up, you would see the list, and you can click to go into a given notebook, maybe duplicate or delete.

In other issues:

• Ability to have multiple sessions. [https://github.com/Multiple notebooks open at once? #9]
• Save/Export/Import format [https://github.com/Save/Export/Import format #10]