AR migration generator includes model's modules in table name. [#4230]

Hello! Could someone review the patch attached to [1] and provide
feedback?

Briefly, it targets rails 3, and fixes "rails g model admin/post".
The generated migration creates a "admin_posts" table while the model
expects a table simply named "posts". With the patch, the migration
creates "posts".

Thank you!
  --phil--

[1] https://rails.lighthouseapp.com/projects/8994-ruby-on-rails/tickets/4230

I've added two alternative patches. The 2nd is a variation on the
first that avoids introducing a new method in NamedBase. The 3rd
leaves NamedBase completely alone, and instead adds
'self.table_name_prefix = "admin_"' to generated models.