Added ShareKit link shortening.

I've added **requiresShortenedURL** to SHKBuffer, because we present the Buffer Sheet straight away for authorisation I've also added the same link shortening code which is called within **promptAuthorization** so links are also shortened when the user hasn't authenticated yet.

Let me know if you have a better method of providing this link shortening even when the sharer is not authorised. I noticed SHKSharer specifically checks that the sheet is authorised before starting the shortening process.

I've also added another option to the settings **bufferShouldShortenURLS** which enables/disables the link shortening contained within the Buffer SDK itself. This uses the users link shortening settings within their Buffer account.

The update also resolves an issue with a crashes caused by the link shortening process within the sheet.