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
