Add basic playlists management

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

features/playlists.feature

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+Feature: Playlists
+
+  So that tracks can be grouped in lists
+  As a listener
+  I want to manage some playlists
+
+  Scenario: List playlists
+    Given Foo and Bar playlists
+    When I am on the playlists page
+    Then I should see "Foo" within "ul>li"
+    And I should see "Bar" within "ul>li+li"
+
+  Scenario: Create playlist
+    Given I am on the playlists page
+    When I follow "Create playlist"
+    And I fill in "Name" with "Electro"
+    And I press "Create"
+    Then I should see "Electro" within "ul>li"

features/step_definitions/playlists_step.rb

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+Given /^Foo and Bar playlists$/ do
+  @foo = Playlist.create!(:name => 'Foo')
+  @bar = Playlist.create!(:name => 'Bar')
+end