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`
since args are parsed afterwards, it may be the case that `NewRequest` should not take args. in fact, maybe it should not take `opts` either, and require explicitly setting them. (would make that 5 arg func nicer)