Add support for Last Played At in the API response

This patch records the last time that the song was played. This allows someone
to write client apps that follow along with what's currently playing

db/migrate/06_add_lastplayed.rb

@@ -0,0 +1,9 @@
+class AddLastPlayed < ActiveRecord::Migration
+  def self.up
+    add_column :songs, :last_played, :timestamp
+  end
+
+  def self.down
+    remove_column :songs, :last_played
+  end
+end

lib/play/app/api.rb

@@ -162,6 +162,7 @@ def music_response(song)
         'artist_name' => song.artist_name,
         'song_title'  => song.title,
         'album_name'  => song.album_name,
+        'last_played_at' => song.last_played
         'song_download_path' => "/song/#{song.id}/download",
         'album_download_path' => "/album/#{song.album_id}/download",
         'alumb_art_url' => song.album.art_url

lib/play/song.rb

@@ -84,6 +84,7 @@ def dequeue!(user=nil)
     def play!
       Song.update_all(:now_playing => false)
       self.now_playing = true
+      self.last_played = Time.now
       votes.update_all(:active => false)
       save
     end