This makes sure there is a GPG signature on any file that is included in
the repo, including APKs, OBB, source tarballs, media files, OTA update
ZIPs, etc.  Having a GPG signature is more important on non-APK files since
they mostly do not have any signature mechanism of their own.

This also adds basic tests of adding non-APK/OBB files to a repo with
`fdroid update`.

closes #232