Namespace model name translations (fixes #58)

* Replace "account" with "pageflow/account" and so on in
  activerecord.yml.
* Also supply aliases for non namespace variant.
  ActiveAdmin skips module names since we register admins with
  `as: "Account"` option to get routing right.
Merge pull request #66 from tf/model-name-translations

Namespace model name translations (fixes #58)