Nested error messages from associations

Now when a nested model has an error, it will be nested in the base
model's error messages hash in addition to the nested model's error
messages hash.

For example, if a user has two posts that are invalid (the first one
lacks a title and the other lacks a body), we get:
```ruby
    {
      :"posts.title" => ["can't be blank"],
      :"posts.body" => ["can't be blank"],
      :posts => [
        { title: ["can't be blank"] },
        { body: ["can't be blank"] }
      ]
    }
```
Instead of just:
```ruby
    {
      :"posts.title" => ["can't be blank"],
      :"posts.body" => ["can't be blank"]
    }
```

This makes it much easier to render errors on a fat client app when dealing with nested models.