Add devise gem.

Using master version at the moment for Rails 4 support.
Ran devise:install generator
Ran rails generate devise User
Add flash to layout.
Add test helpers to RSpec.
Add default URL options to emails.
Users should only see their own bookmarks.
Fill out User Factory.
Add index action, route.
Teach User about bookmark association.
Fixed spec.
Add user association.

Went ahead of myself here; added user association to other tables as
well.
Added index template.  Contains a User's bookmarks.
Site index should only display a user's site.
Controller only gets the user's sites.
Teach User model about Site association.
Suppress warning by not eager loading.
Only find a user's bookmarks
Controller selects only a user's bookmarks.
Put login/user creation before each step.