Update models/ion_auth_model.php

Add a ```LIMIT 1``` to the ```email_check()``` and ```username_check()``` functions because we only need to find one row

Currently:

    return $this->db->where('email', $email)
		            ->count_all_results($this->tables['users']) > 0;


Update:

     return $this->db->where('email', $email)
                 ->limit(1)
		         ->count_all_results($this->tables['users']) > 0;