Add support for once persist many embedded documents with parent document

Before you only can once persist on embedded document with parent document.
Now add support for can once persist many embedded documents with parent documents.
```ruby
class User
  include Mongoid::Document
  embeds_many :items  
end

class Item
	include Mongoid::Document
	field :name,type:String
	embedded_in :user
end
# you can persist many embedded documents such as 
User.create.items.create([{:name => 'foo'},{:name => 'bar'}])
User.create.items.create!([{:name => 'foo'},{:name => 'bar'}])
```