Support for cyrillic letters in slugs (while in :ru locale)

I've fixed an issue occured with empty post slugs (causing routing errors, #217) for posts with titles consisting totally of cyrillic characters.

I've added gem `russian`, which activates correct cyrillic processing when `I18n.locale = :ru`.
Nethertheless, it breaks classical ASCII transliteration, so I've messed with `I18n.locale` variable in `slug_spec.rb` a little bit.