Update specs to remove deprecation warnings

Test schema is now maintained automatically from spec_helper.
Update specs to the new 'expect' syntax, removing occurences of
'should's and 'stub's.
Fix User model
Fix devise configuration
Authorize for categories controller
Improve categories controller error handling

When a non-admin user wants to access admin actions, an error message
will be shown to them, instead of redirecting to login page (they are
signed in after all, but without admin permissions)
Finish Product model
Finish Category model
Finish Review model
Finish review decorator
Hide not allowed actions from views
Product controller
Add seeds
Start design
Category and Product views
Category and Product views
User and Review views
Product and Review views
Devise views and other small changes
Prepare for initial deploy to heroku
Fix bug not saving user with the product