Support symmetric passphrase-only encryption

AFAIK duplicity uses symmetric encryption when you pass it a passphrase but not a GPG key.

Simply specifying `backup_gpg_pw` and leaving `backup_gpg_key: disabled` (the default) enables this behavior.
Merge pull request #4 from klaemo/patch-1

Support symmetric passphrase-only encryption
Update contributors
Bump version: 1.2.2 → 1.2.3
Merge branch 'develop'