Allow setting of class on association

Given the model classes

```
class User < ActiveRecord::Base
end

class GenericEngine::User < ActiveRecord::Base
  table_name = 'user'
end

class Post < ActiveRecord::Base
  belongs_to :user, class_name:  GenericEngine::User
end
```

This pull requests adds the following syntax:
```
FactoryGirl.define do
  factory :user

  factory :post do
    user class: GenericEngine::User
  end
end
```

This allows support for sharing database tables across engine models.  Specifically to combat the occurrence of the error:

```
ActiveRecord::AssociationTypeMismatch:
       GenericEngine::User(#70355900521280) expected, got User(#70355900924220)
```

when you do:

```
create(:post)
```
or
```
user = create(:user)
create(:post, user: user)
```