Adding I18n

Added a config/locales/en.yml (and also a pt-BR.yml) file, moved a lot of hardcoded strings into it and made references to them using Rails's I18n. Also changed a few date's strftime to use I18n.l method.
Rails already adds i18n since 2.x I believe. So unless it is to be used by something else like sinatra, it doesn't needs to be specified on gemspec.