Load phpCAS Library Based on class_exists( )

Load the phpCAS library from the $gfedf_phpcas_path configuration
variable path if class_exists( ) fails to find the class name 'phpCAS'
already defined in the environment. Also call the phpCAS::client method
if class phpCAS does not exsist and assume that phpCAS::client has
already been called if the class does exsist.