At product page, display message if the product has no items.
Add link from product page to wabpage.
Rename constant APP_CONFIG -> SITE_CONFIG and also config file's name.
Read site title from site_config.yml.
Generate scaffold of category.
db/schema.rb: Update.
db/seeds.rb: Add data for category.
Generate migrationfile to add category_id to items table.
db/schema.rb: Update.
Make association between item and category.
Display category in item page.
Edit category in item edit page.
app/views/items/show.html.erb: Fix bug.
app/views/items/show_not_signed_in.html.erb: Fix bug.
app/models/{brand,product}.rb: Fix typo.
Add category sellect field in new item page.
Show items that user's only, in item index page.
app/views/items/new.html.erb: Display product name.
Validate item model.
Revert "Validate item model."

This reverts commit af9b057bb86bec550d448fa308493fc75cde32ca.