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`
note that it isn't exiting in the `if err == io.EOF {` case. which is sketch? Oh, file may be non-nil (sometimes io readers return `(lastThing, io.EOF)`) hm. still think there is an "early error" out of here.