accepts_nested_attributes_for

This PR adds support for the accepts_nested_attributes_for method which you probably already know from Rails' ActiveRecord: http://api.rubyonrails.org/classes/ActiveRecord/NestedAttributes/ClassMethods.html#method-i-accepts_nested_attributes_for

This allows something like:
``` ruby
class Foo::User
  include Her::Model

  has_one :pet
  accepts_nested_attributes_for :pet
end

user = User.new
user.pet_attributes = { :name => "Rodriguez" }
user.pet.name // "Rodriguez"
```

And forms like:
``` erb
<%= form_for @user do |user_form| %>
  <%= user_form.fields_for plug do |pet_form| %>
    Pet's name: <%= pet_form.text_field(:name) %>
  <% end %>
<% end %>
```