Extracted attributes assingment from ActiveRecord to ActiveModel

Now you are able to do:


``` ruby
class Feedback
  include ActiveModel::AttributeAssignment
  attr_accessor :email, :subject, :body
end

f = Feedback.new
f.assign_attributes(
  :email => "bogdan@example.com", 
  :subject => "Hello", 
  :body => "The app dont work for me"
)
```

This is just a proof of concept PR.

There is still some todos:

* Write tests for ActiveModel::AttributeAssignment
* Deprecate AR::UnknownAttributeError in flavor of AM one
* Add change log entry
* rebase change log entry 2-3 times

I am gonna add them if this will be approved to merge in.