Added album 'buy' buttons in music page.

Added album 'buy' buttons in music page.
Also made general improvements.
    application/models/header_model.php
    application/models/music_model.php
    application/views/pages/buy.php
    css/music.css
    css/style.css
    images/icons/br_down_16.png
    images/icons/br_up_16.png
    javascript/buy.js