Add task to show migration status

This is similar to rails task "db:migrate:status".
The output would be like:

```
 Status   Migration ID    Migration Name
--------------------------------------------------
   up     20130101000000  Create sites
   up     20130102000000  Create bookmarks
  down    20130201000000  Add score to sites
```

There is one issue. `rake groonga:setup` only records the latest migration ID in `schema_migrations` table, so only the latest migration would be "up" and other would be "down" even they would be migrated after executing `rake groonga:setup`. I would like to resolve this issue later on.