Added preferences to use non-default keyrings (e.g. for development)

Please test this. You can generate a new pair of keyrings using the following command:

    gpg --no-default-keyring --keyring debug-pub.gpg --secret-keyring debug-sec.gpg --gen-key

This will create the keyrings `debug-{pub,sec}.gpg` in your `~/.gnupg/` folder. If you enter these file names or absolute paths in the settings, the keyrings will be found by gpg. I recommend creating an alias for running debug commands on the debug keyrings, e.g.

    alias gpgdebug='gpg --no-default-keyring --keyring debug-pub.gpg --secret-keyring debug-sec.gpg"