support recursive folder downloads

[mcginty]

I think this should be in Protocol v0.

Right now, the protocol is super simple: the client sends a version byte (arguably unnecessary since that can be included in the command to the server over SSH) and an offset (for resuming a previously dropped session at the right point).

The header should probably look more like

[ number of files remaining | first file header ]

where file header is

[ file name | file size remaining ]

in single file transfer mode, the simplified header of just sending the bytes remaining currently in use should still be fine.