Update user_model for compatibility with MY_Model changes in e77ea0aa9be4f48cf38cd2c5f5d81e52b15429a1

Additionally:
- cleaned up PHPDoc comments in MY_Model.php
- removed unused $return_type argument from BF_Model::find() method
- refactored BF_Model::delete() and BF_Model::delete_where() to show the
similarity in their logic (should probably make one a wrapper of the
other)
- added @internal note to BF_Model::count_all() regarding potential
confusion with $this->db->count_all()