Fixed "sudo -n" and "sudo -v" for OS X Mavericks

On OS X 10.9 Mavericks, the command "sudo -n" return 0, even if the sudo session is not alive.
Replaced by "sudo -v -p", with a custom message.
Updated "NEWS" file with #84 (Fixed "sudo -n" and "sudo -v" for OS X Mavericks)
Merge pull request #84 from CentralReport/sudo-mavericks

Fixed "sudo -n" and "sudo -v" for OS X Mavericks