improving language chooser

Related to https://github.com/coopdevs/timeoverflow/issues/92

![captura de pantalla 2014-12-03 a la s 01 41 31](https://cloud.githubusercontent.com/assets/576701/5273884/9618674c-7a8d-11e4-9016-d1f1707c4fa2.png)

```ruby
link_to switch_lang_path(locale: locale) do
  concat t(locale_key, locale: locale)
  concat " (#{t(locale_key)})" unless I18n.locale == locale
end
```