add check if CMB_PATH and CMB_URL are already defined

allows the user to simply include a custom value in their functions.php in case it is being included in a symlinked directory:
define( 'CMB_URL' , get_template_directory_uri() . '/library/Custom-Meta-Boxes/' );
Merge pull request #103 from kaymmm/master

add check if CMB_PATH and CMB_URL are already defined