Multipart File Transfer

This PR is ready for CR.

Overview of changes:

* Added a `commands.File` interface that implements `io.Reader` (which also has a filename and can be read in parts)
* Changed from accessing file argument values in `Request#Arguments()` to a separate `Request#Files()`
* Open files recursively on the client (and make them into `commands.File`s)
* Transfer files as multipart in `commands/http`
To the reader, `FilesystemObject` makes it sound a lot more complicated than it is. And anyway, it is called a "filesystem", not a "file-and-directory-system". You're right, the people who defined the original terms didn't do it optimally. But I still think `File` is the best choice.