Multipart File Transfer

This PR is ready for CR.

Overview of changes:

* Added a `commands.File` interface that implements `io.Reader`, 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`