Require login to access interior parts of the site

Also, required logged out state to access sign in/sign up forms.
Generate skeleton for Notebook model
Migrate database for Notebook model
Add validations for Notebooks
Associate Notebooks to Users
Create a default starting notebook for new users

Happens upon user creation. Required User.notebook_id to be mass
accessible.
Generate skeleton for Note model
Migrate database for Note model
Associate Note model to notebooks & users
Add routes for notebook and notes

Also eliminated Users#show as it will be replaced by one of these new
routes.
Generate skeleton for notebook & note controllers
Remove other references to Users#show

Notebooks#index will be taking its place in a moment.
Add two spec files missed in a prior commit

These were generated when the notebooks and notes controllers were
generated.
Create view for Notebooks#index

Log in and user creation will (temporarily) redirect to this route on
success.
Add form to create or update a notebook

This does not yet include functionality for changing the user's default
notebook.
Build route to delete notebooks

This will throw an error if a user tries to delete her last notebook
and will also reassign the user's default notebook if she deletes her
current default.

Also took the oppotunity to add an association for the default notebook.
This is not currently used, but will likely come in use later.
Allow the user's default notebook to be changed

This required new routes to draw a distinction between simply renaming a
notebook versus editing its properties.
Add Notes#index view

This intelligently determines whether to show the notes for a particular
notebook or all notes based on a query string embedded in links on the
Notebooks#index view.
Enable note creation

Notes are always instantiated as untitled and otherwise empty. The user
can then edit them later.
Change implementation of notes index

Modified routes so that Notebooks#show and Notes#index share the burden
for showing lists of notes. Before, this was all done under Notes#index,
but that was getting unwieldy.

This change also required a before filter to keep users from modifying
or viewing other users' notebooks.