Implement :hash_key => true

I've been using the AWS SDK for accessing dynamoDB and I noticed that if the hash key on my table was named something besides "id" the .each iterator for scopes wouldn't work, instead raising errors about the hash_key being missing in the put_item response.

I put together this quick hack that lets users do
```ruby
string_attr :foo, :hash_key => true
```
and have that value act as the hash_key for the table.