Support for paper_trail custom version classes

Tweak paper_trail/auditing_adapter to provide support for using custom version classes/models/db tables. Example from paper_trail's wiki:
```
class PostVersion < PaperTrail::Version
  # custom behaviour, e.g:
  self.table_name = :post_versions
end

class Post < ActiveRecord::Base
  has_paper_trail :class_name => 'PostVersion'
end
```