Add SFTP-only backend

[Cristian Klein]

As of v0.6.0 duplicity does not have a pure SFTP backend, i.e. a backend that uses *only* the SFTP command to communicate with the backup host. The existing backends (SSH / SCP) use SFTP but also SCP. Using only SFTP is desired, because one may create a restricted shell in which SFTP can be run without shell (PTY) access. This is not possible with SCP.

The patch below addresses this issue, by adding a "sftp://" scheme. The most important file (backends/sftpbackend.py) is a modification of sshbackend, but replaces all scp commands with their equivalent sftp ones.

[Kenneth Loafman (kenneth-loafman)]

I fixed one problem with arguments to get and patched duplicity to be sftp only. Clean solution.