Bandcamp: Switch to esc_attr instead of int for album and track IDs

Bandcamp Album and track IDs can be larger than `PHP_INT_MAX`, thus causing issues in embeds.

View original trac ticket here:
http://plugins.trac.wordpress.org/ticket/2051
Bandcamp shortcode: check if track and album IDs are numbers

See #55
Merge pull request #55 from jeherve/2051-trac

Bandcamp: Switch to esc_attr instead of int for album and track IDs