Made requested corrections

Hey, sorry it took me so long to get back to this, work had me a little busy the past couple weeks. I made all the corrections that you requested. The only comment I had is in the config/routes.rb file. The Playlists#add_song action is mapped to the add_song_playlist_path(@playlist) helper, rather than playlist_add_song_path.

I've been looking in to cancan and believe that with some experimenting, I could implement authorization for all the different actions, if you haven't already begun working on it.

P.S. Sorry if this pull request isn't as concise as it could be, I've never issued one of these before!