#18 added primary key to `id`

Added a Primary key on the `id` column when creating new tables.
```
$sql = "CREATE TABLE " . $table_name . " (
	                id mediumint(9) NOT NULL AUTO_INCREMENT,
	                token tinytext NOT NULL,
	                data text NOT NULL,
	                PRIMARY KEY (`id`),
	                UNIQUE KEY id (id)
	                );";
```