support for dependent: :destroy, resolves #34

Hey. I picked this one to start off with since it seemed like a simpler one. Let me know if you think this approach is suitable, or if there's an easier way of getting the keys for an association that I'm missing. There are no checks on this, so currently if you write:

```ruby
has_many :comments, dependent: :foo
```

it will just do nothing. If you'd rather that raise an error, I'm happy to add that.