This permits external packages implementing e.g.
FTP or gopher to register themselves with the
http.DefaultClient:

package ftp
func init() {
    http.DefaultTransport.RegisterProtocol("ftp", &ftp{})
}

Client code would look like:

import (
    _ "github.com/exampleuser/go/gopher"
    _ "github.com/exampleuser/go/ftp"
)

func main() {
    resp, err := http.Get("ftp://example.com/path.txt")
    ...
}

R=dsymonds, rsc
CC=golang-dev
https://golang.org/cl/4526077