Added Bands model

[+] name:string
[+] genre:string
[+] has_and_belongs_to_many :users
Added Album model

[+] name:string
[+] genre:string
[+] album_art_url:string
[+] Relationship: belongs_to :band
Added Song model

[+] name:string
[+] bpm:string
[+] segments_per_line:integer
[+] Relationship: belongs_to :album
Fixed the bands_users relationship table.