Feature request : Ability to customize the User class

[bricesanchez]

Hello !

I'm using this gem in an namespaced app, my devise User is Portal::User, so i've to define the class_name of the belongs_to :user :
https://github.com/HeHStudy/surveyor/blob/master/lib/surveyor/models/response_set_methods.rb#L11

Could we find a clean way to add config in a initializer to be able to define the class_name like this ?

# lib/surveyor/models/response_set_methods.rb
[...]
belongs_to :user, class_name: Surveyor.user_class
[...]