debian: Add watch file and public signing key.

debian: Add watch file and public signing key.

The watch file is an adjusted version of the "direct access to the
git repository (tags)" example of uscan(1) (part of the devscripts
package), which is appropriate for tfortune since the project does
not publish release tarballs.

The key file was created with

	gpg --export --export-options export-minimal --armor \
		87B45FDC6D91B2CA5BEAAB545ADA35403100930F > signing-key.asc

Suggested-by: lintian(1)
Reviewed-by: Andreas Metzler <ametzler@bebt.de>