add support to generate EntityDescriptor ID on the fly closes #71

closes #71

right now onthefly value can be generated (for local managed entities) using custom function:

you need:
create file valid php file "custom_helper.php" in application/helpers
this file is loaded dynamically if found

in this file you need to create function customGenerateEntityDescriptorID : 
###

function  customGenerateEntityDescriptorID($arg)
{
   /**
    * some code
   */
  return $result;
}

###
this function receivers $arg as array : 
$arg = array(
         'id'=>ID_OF_ENTITY_FROM_DB,
         'entityid'=>'ENTITYID'
)

I thinks ti's enough information to generate id