Missing indexes helper

ping @tjoyal @sgnr 

```ruby
RescueFromDuplicate.missing_indexes
=> [#<RescueFromDuplicate::MissingIndex:0x007fc6f69d2938
  @attributes=[:title],
  @columns=["shop_id", "title"],
  @model=
   ShopPolicy>,
...
```
I'm saying the title of [this article](http://dev.mysql.com/doc/refman/5.0/en/mysql-indexes.html) doesn't mention indices