Added support for a selectbox/drop-down field type.

Usage: Fields can now take a fourth (optional) parameter during
register() calls.  This parameter, indexed as 'meta', is an array.  For
selectboxes, it's implemented to use key/value pairs (i.e.,
$my_options['USA'] = 'United States';).

Sample usage inside a register() call:
$fields  = array( array( 'name'  => 'lang',
'type'  => 'select',
'label'  => 'Language',
'meta'  => array('en' => 'English', 'es' => 'Spanish', 'fr' => 'French')
));
$args  = array( 'label'         => 'Files',
'post_type'     => array( 'page' ),
'filetype'      => null,
'note'          => '',
'button_text'   => __( 'Attach Files', 'attachments' ),
'modal_text'    => __( 'Attach', 'attachments' ),
'fields'        => $fields);
$attachments->register( 'my_custom_instance_name', $args );