Pulling backups over sftp

[andrewchambers]

I realized our new vfs abstraction could also be applied to the 'put' code. This would let us backup anything implementing the vfs interface. If we get a corresponding sftp vfs then pull based backups should become trivial.

It will be interesting to see how it would perform.

example:

bupstash put sftp://[e-mail]/path/to/something