Add basic playlists management

* Index of playlists
* Create a new playlist
* Basic playlist model with mandatory name

app/views/playlists/index.html.haml

@@ -0,0 +1,4 @@
+= link_to 'Create playlist', new_playlist_path
+%ul
+  - @playlists.each do |p|
+    %li= p.name

app/views/playlists/new.html.haml

@@ -0,0 +1,4 @@
+= form_for @playlist do |f|
+  = f.label :name
+  = f.text_field :name
+  = f.submit 'Create'