Fix to Issue #46 - Factory.php attempting to create relationship for fields with {$class}_id

Factory.php was not taking into account fields that have `{$class}_id` as their primary key instead of `id` eg `user_id` as the primary key for the `users` table. - fixed and tests still pass.

All existing test passing but sorry - I'm only half way into "Laravel Testing Decoded" so not confident enough to write additional test as I get to the
if (static::$isSaving and preg_match('/([A-z]+)[-_]id$/i', $field, $matches))
line (c. 330) and I'm not sure you want to set the $isSaving to true & start saving to a db.