#71 Add support for GPG-encryption

[Gert-Jan]

Is it possible to add support for GPG-encryption? Currently the script only supports symetric encryption of the dumps with openssl. This has the drawback that the encryption/decryption password has to be stored in a configuration file on the server.

With GPG the encryption can be done with a public key, so the decryption key can be kept somewhere safe (not on the server that creates the backup).