Implement sh.mv()

[humphd]

We need an implementation of the mv command on FileSystemShell, with tests and docs in the README. It should work in similar ways to the unix mv command, and do the right thing with files vs. dirs.

[sedge]

@modeswitch The mv utility will automatically overwrite files during the move unless told not to. Do we want to keep this for our version?

[modeswitch]

@sedge Yes, that sounds reasonable.