Add option 'class_namespace' to association macros

Can be used as `has_many :languages, class_namespace: 'ProfileData'` to get ProfileData::Languages class, but this option mainly added to support this variant:

```ruby
class Profile < ActiveRecord::Base
  with_options class_namespace: 'ProfileData' do |m|
    m.has_many :languages
    m.has_many :education
    m.has_many :interests, dependent: :destroy
  end
end

class ProfileData::Language
  # ...
end

class ProfileData::Education
  # ...
end

class ProfileData::Interest
  # ...
end
```

Class structure like this is common for big projects, so I think it can be used to make code a little cleaner.

I will handle changelog and doc changes if you support this changes.