Implemented support for the new ShortUrl endpoints

This PR lands support for the new ShortUrl endpoints. Introduced methods:

 - `getShortUrl(imageIdentifier, callback)`
 - `deleteAllShortUrlsForImage(imageIdentifier, callback)``
 - `deleteShortUrlForImage(imageIdentifier, shortUrl, callback)`

Documentation has also been updated to reflect the changes, and a new `ShortUrl`-class has been introduced.