Add SemVer 2.0.0 italian translation

This is the italian translation of Semantic Versioning 2.0.0.

With this translation comes little improvements in language support.

Accordingly to [The Internationalization Best Practices: Specifying Language in XHTML & HTML Content](http://www.w3.org/TR/2007/NOTE-i18n-html-tech-lang-20070412/) I've improved support language in `_layouts` files:
* adding `lang` and `dir` attributes to `html` tag
* adding `lang`, `rel` and `hreflang` attributes to the `a` tags that contains a language name
* making a trade off between [Best Practice 8: Handling attribute values and element content in different languages](http://www.w3.org/TR/2007/NOTE-i18n-html-tech-lang-20070412/#ri20050128.175100333)  and [not annoying translators with name language](https://github.com/omasanori/semver.org/commit/7626a17c6986a47eb383d591869f4680c133ea53#commitcomment-3736397) achieving a solution like the one on the top of the page at [gnu.org](http://www.gnu.org/) with language code after the localized language name.

Thanks to @nafigator and @omasanori for feedback.

Let me know if you like these new little improvements.

Thank you!