add index index_taggings_on_taggable_id(taggable_id ...) for acts-as-taggable-on

Fix Problem4 of #61. 

I ran `mysql> show full processlist` when CPU usage of mysqld is high.

```
[01:43:08 root@localhost/(none) :3] show full processlist;
| Id        | User | Host      | db             | Command | Time | State        | Info
| 875146786 | root | localhost | yohoushi       | Query   |    0 | Sending data | SELECT `tags`.* FROM `tags` INNER JOIN `taggings` ON `tags`.`id` = `taggings`.`tag_id` WHERE `taggings`.`taggable_id` = 18687 AND `taggings`.`taggable_type` = 'Node' AND (taggings.context = 'tags' AND taggings.tagger_id IS NULL) |
```

Explain the query. Number of rows is pretty large. It seems that index is not working well.

```
[02:46:09 root@localhost/yohoushi :6] explain SELECT `tags`.* FROM `tags` INNER JOIN `taggings` ON `tags`.`id` = `taggings`.`tag_id` WHERE `taggings`.`taggable_id` = 18687 AND `taggings`.`taggable_type` = 'Node' AND (taggings.context = 'tags' AND taggings.tagger_id IS NULL) ;
+----+-------------+----------+--------+---------------+--------------+---------+--------------------------+--------+--------------------------+
| id | select_type | table    | type   | possible_keys | key          | key_len | ref                      | rows   | Extra                    |
+----+-------------+----------+--------+---------------+--------------+---------+--------------------------+--------+--------------------------+
|  1 | SIMPLE      | taggings | index  | taggings_idx  | taggings_idx | 1938    | NULL                     | 887571 | Using where; Using index |
|  1 | SIMPLE      | tags     | eq_ref | PRIMARY       | PRIMARY      | 4       | yohoushi.taggings.tag_id |      1 |                          |
+----+-------------+----------+--------+---------------+--------------+---------+--------------------------+--------+--------------------------+
2 rows in set (0.00 sec)

[02:50:16 root@localhost/yohoushi :5] show create table taggings;
...
  PRIMARY KEY (`id`),
  UNIQUE KEY `taggings_idx` (`tag_id`,`taggable_id`,`taggable_type`,`context`,`tagger_id`,`tagger_type`),
...
```

Created another index not including `tag_id`. Now, mysql processes WHERE first, then JOIN, which is much efficient in this case.

```
[02:50:08 root@localhost/yohoushi :4] alter table taggings add index index_taggings_on_taggable_id(taggable_id, taggable_type, context, tagger_id, tagger_type);

[02:50:08 root@localhost/yohoushi :4] explain SELECT `tags`.* FROM `tags` INNER JOIN `taggings` ON `tags`.`id` = `taggings`.`tag_id` WHERE `taggings`.`taggable_id` = 18687 AND `taggings`.`taggable_type` = 'Node' AND (taggings.context = 'tags' AND taggings.tagger_id IS NULL);
+----+-------------+----------+--------+--------------------------------------------+-------------------------------+---------+--------------------------+------+-------------+
| id | select_type | table    | type   | possible_keys                              | key                           | key_len | ref                      | rows | Extra       |
+----+-------------+----------+--------+--------------------------------------------+-------------------------------+---------+--------------------------+------+-------------+
|  1 | SIMPLE      | taggings | ref    | taggings_idx,index_taggings_on_taggable_id | index_taggings_on_taggable_id | 1165    | const,const,const,const  |    5 | Using where |
|  1 | SIMPLE      | tags     | eq_ref | PRIMARY                                    | PRIMARY                       | 4       | yohoushi.taggings.tag_id |    1 |             |
+----+-------------+----------+--------+--------------------------------------------+-------------------------------+---------+--------------------------+------+-------------+
2 rows in set (0.00 sec)
```