Add support for signing the Release file when verifying.

Hey there,

I noticed that the --sign option wasn't valid when using the 'verify' command after removing a package. Since the default behaviour without --sign is to remove the Release.gpg file, this then causes Apt to show the following error when attempting to install from the repository:

'WARNING: The following packages cannot be authenticated!"

I made --sign a class_option, rather than duplicating it in each command, but feel free to adjust as you see fit :)

Thanks!