[Plugin: Update Notifier] blogname / blog url at the subject of an email

Because it is so varied as to what people want for the perfect email subject I added a filter to it, usage is something like:

add_filter( 'updatenotifier_subject', 'my_custom_subject' );
function my_custom_subject( $s ) {
    return '[' . get_bloginfo( 'name' ) . '] Updates available';
}