Generate scaffold for tags.
db/migrate/20141230233607_create_tags.rb: Edit (remove timestamps).
Generate scaffold for taggings.
db/migrate/20141230234524_create_taggings.rb: Edit.
db/schema.rb: Migrate.
Make association: wallpapers, tags and taggings.
app/models/wallpaper.rb: Implement add_tag and add_tags.
app/views/wallpapers/show.html.erb: Display tags.
Display tags field at new wallpaper page.
Tagging at new wallpaper page is available.
app/models/wallpaper.rb: Implement delete_tags and update_tags.
Update edit wallpaper page.
Editing wallpaper is available.
app/views/images/new.html.erb: Update.
Merge branch 'feature-tagging'
app/models/wallpaper.rb: Fix bug of multiply tagging.
Merge branch 'feature-tagging'