Add new cask for Prey project.

This cask install the batch version of Prey package, as detailed in:
http://support.preyproject.com/kb/installation/how-to-deploy-prey-in-batch-mode-mac-os

As it requires an API key to be installed, a warning is displayed to the
user with an explanation on how to fix the issue.

Also added -E option to sudo invocation so environment variables can be
passed to the installer.

closes #953

Signed-off-by: phinze <paul.t.hinze@gmail.com>