added artist page
HTMLで記述されていた部分が残ってたので、slimに修正
modified:   app/views/artist/show.html.slim
refs  #40
* 不要なrequireを削除
* 不要な";"を削除
* URL設計の見直し(artist→artists)
* 埋め込みyoutubeのサイズを固定からウインドウ枠いっぱいを使うように変更

modified:   app/assets/stylesheets/artist.css.scss
modified:   app/controllers/artist_controller.rb
modified:   app/views/artist/show.html.slim
modified:   config/routes.rb
* URL設計の見直し(artists/:artist_id => artists/:path_key)

modified:   app/controllers/artist_controller.rb
Merge pull request #57 from i-d-e-a-t/add-artist-page

added artist page