Sets a variable containing the locale

With this commit you can retrieve in the views the active locale. Useful when using `globalize_fields_for_locales` to show the locale of the input:

```ruby
<% f.globalize_fields_for_locales [:en, :it] do |g| %>
  <legend>Locale: <%= g.locale %></legend>
  <%= g.input :name" %>
<% end %>
```