Added option to customize the URL path

[kalicki]

Example:

// Initialize the metabox class
add_action( 'init', 'be_initialize_cmb_meta_boxes', 9999 );
function be_initialize_cmb_meta_boxes() {
    // Custom path URL
    define( 'CMB_META_BOX_URL', trailingslashit( CUSTOM_URL_PATH ) );
    if ( !class_exists( 'cmb_Meta_Box' ) ) {
        require_once( 'lib/metabox/init.php' );
    }
}