Translate into it_IT

[LorenzoRogai]

Pull request for the translation of the tips

Done:

• 2015-12-29-inserire-un-elemento-in-un-array.md
• 2016-01-01-angularjs-digest-vs-apply.md
• 2016-01-05-differenze-tra-undefined-e-null.md
• 2016-01-13-consiglio-per-misurare-le-prestazioni-di-un-blocco-di-codice-javascript.md
• 2016-01-19-concatenazione-stringhe-sicura.md
• 2016-01-20-restituire-oggetti-per-permettere-il-concatenamento-di-funzioni.md
• 2016-01-21-mescolare-un-array.md
• 2016-01-23-metodo-veloce-per-convertire-stringhe-in-numero.md

Unfinished:

• 2016-01-03-migliorare-condizioni-nidificate.md

[loverajoel]

@LorenzoRogai awesome! 1 - Please add a nice description to this PR :dancer: 2 - I will need your help here (I mentioned you)