ADD MusicAlbum and Genre files

[emmanuellmhango]

I have added the following:

1. music_album.rb implementing MusicAlbum class
2. genre.rb implementing Genre class. Implement methods:
   • [ ] add_item method in the Genre class
   • should take an instance of the Item class as an input
   • should add the input item to the collection of items
   • should add self as a property of the item object (by using the correct setter from the item object)

• [ ] can_be_archived?() in the MusicAlbum class
• should override the method from the parent class
• should return true if parent's method returns true AND if on_spotify equals true
• otherwise, it should return false