Support for creating proxy extensions from several modules

Make it possible to extract the extensions methods to more them one module:

```ruby
class Person
  include Mongoid::Document
  embeds_one :name, extend: [ Name::Extension, Name::AnotherExtension ]
end
```

Note: API like [association extensions](http://guides.rubyonrails.org/association_basics.html#association-extensions) from ActiveRecord.