I18n inheritance

This pull requests adds support for I18n model inheritance. Example setup;

```
en:
  formtastic:
    labels:
      group:
        title: "Your Title"
```

```ruby
class Group < ActiveRecord::Base
end
```

```ruby
class Company < Group
end
```

```
<%= semantic_form_for Company.new do |f| %>
  <%= f.inputs do %>
    <%= f.input :title %>      # => :label => "Your Title"
  <% end %>
<% end %>
```