Mix ActiveModel::AttributeAssignment into Base

[seanpdoyle]

By including the ActiveModel::AttributeAssignment, the Base class
can access the assign_attributes method for bulk assignment of
attributes without saving them to the server (like through
Base#update_attributes).

Person.schema = { name: "string" }

person = Person.new
person.id   # => nil
person.name # => nil

person.assign_attributes id: 1, name: "Matz"
person.id   # => 1
person.name # => "Matz"

[seanpdoyle]

After more consideration, I think the similarities that the between assign_attributes and load have the potential for confusion. I'm closing this PR in favor of #410, since that explores fully integrating with Active Model attributes.