Commit accidentally missed files, fix #15.
Merge remote-tracking branch 'upstream/master'
passing Exercise_5_Test.useDefaultMethodToReturnPlaylistOrderedByArtist
passing Exercise_5_Test.overridesDefaultMethodInClassToProvideCustomSongRatingAlgorithm
passing Exercise_5_Test.overrideDefaultMethodInInterfaceToProvideUserEnteredSongRatings