add short_url parameter to the shares method of DropboxClient

Currently the "shares" method in Dropbox Ruby SDK doesn't provide option to specify whether the returning share link to be short URL or full length. This prevents Ruby developers from modifying the share link, like add "?dl=1" parameter to get a direct download link of the sharing item.

This pull request solves the problem by providing a "short_url" parameter( default to "true") to the "shares" method, which allows users to specify the format of the returning share link.