Fixes the non-working SSH-with-password connection

A possible bug (see https://bugs.launchpad.net/duplicity/+bug/1190724) makes the SSH connection with a password fails.
By using "--ssh-askpass" option when FTP_PASSWORD is set, we force duplicity to use that environment variable.

Closes: #10
Increments version to 0.1.5.
Merge pull request #11 from C-Duv/bugfix/10-cannot_connect_to_ssh_with_password

Bugfix/10 cannot connect to ssh with password