Add internationalization support

Thanks to that idea and realization:
https://github.com/onemanstartup/middleman-blog

Now you can use frontmatter or path's ":lang" part for language indication. Also you can use local_articles helper in templates to select only articles with selected language.
While creating article with rake you can pick a lang option.
Tests are provided. I haven't much cucumber experience, but it seems, it works. Sorry, if I have a mistakes. Also sorry for my bad English very much.