add controller for Artists
change routes for artists
add controller albums
add controller for genres
add controller media_types
add controller playlists
add controller tracks
add nested routes
add json api for artist index and show action
add pry and better_error gem
add create action for artists
move 'record not found' method to application controller
customize json format for album model
add index and show action for albums
get albums by artist in action 'index' for albums_controller
add action 'index' and 'show' for tracks_controller
change wording when record not found
add route for searching tracks by genre and media type
remove model contact
add model user