Support "Create/Drop index" statements

Please add support for [create index](http://www.1keydata.com/de/sql/sql-create-index.php) statements, dropping as well.

```
  CREATE  INDEX "index_posts_on_comment_count" ON "posts" ("comment_count")
  DROP INDEX "index_posts_on_comment_count" ON "posts"
```

ActiveRecord migrations use this syntax to add indices.
